If you look up the question "how many wives did Muhammad have?", you may well find the answer. But in spite of it being such a simple question, in need of no further clarification, what you will also find is a long list of defences of Muhammad's character.
Generally speaking, we in the Western world find polygamy repugnant. It is morally questionable for a man to have so many wives, to the extent where even many unbelievers, who today are opposed to most natural elements of marriage, still use polygamy as an argument against Islam, and even Judeo-Christianity.
In general, moral objections to any religion are flawed. If Islam is true, and Allah saw fit to allow Muhammad the right to marry 9 women at once, then Muhammad was guilty of no crime. If Islam is not true, then the religion that is true must judge Muhammad. But the truth of that religion must first be established in order for us to use it, and it is therefore a bad argument to use against Islam.
Ironically, Christians do not have this problem, as Muhammad himself confessed the truth of the Christian scriptures. In fact, he claimed to be in the same vein as the Biblical prophets, including Jesus, who, according to Islam, is merely a prophet, and not the Son of God. Because Muhammad affirmed the truth of the Christian scriptures, he can be judged according to Christian morality, and his prophethood stands or falls upon it.
But polygamy (unlike pedophilia) is not unique to Muhammad. Thus, Islamic defences of Muhammad typically show that Old Testament prophets also had more than one wife. This is actually true. In the Old Testament, God was a lot more lenient with marriage than He is today. For example, in the law of Moses, God permitted divorce. But here's what Jesus had to say on that issue:
"The Pharisees also came to Him, testing Him, and saying to Him, “Is it lawful for a man to divorce his wife for just any reason?” And He answered and said to them, “Have you not read that He who made them at the beginning ‘made them male and female,’ and said, ‘For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh’? So then, they are no longer two but one flesh. Therefore what God has joined together, let not man separate.” They said to Him, “Why then did Moses command to give a certificate of divorce, and to put her away?” He said to them, “Moses, because of the hardness of your hearts, permitted you to divorce your wives, but from the beginning it was not so. And I say to you, whoever divorces his wife, except for sexual immorality, and marries another, commits adultery; and whoever marries her who is divorced commits adultery.”" (Matthew 19:3-9).
From this, we see that God did not instantly descend upon Israel and enforce perfection. In fact, if He dealt with Israel the way in which He will finally deal with humanity, the nation would not survive. As Paul tells us, "For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord." (Romans 6:23). Now, if the wages of sin is death, why are only some sins met with capital punishment in the law? Because God met Israel where they were at. This, sadly, was a dirty place. And so God made certain concessions, which He tightened up significantly when Christ came.
As it stands, the New Testament tells us marriage is based on Adam and Eve. For this reason, we can say that marriage is a covenant between one man and one woman. This is so firm a standard that in the New Testament, people in prominent church leadership roles were only permitted one wife (1 Timothy 3:2, 12; Titus 1:5-6).
Thus, we see that Muhammad fails to keep with the historic progression of "...the faith which was once for all delivered to the saints." (Jude 1:3). How, then, can he be considered a prophet?
But there is more than this. See, in Christianity, "...all have sinned and fall short of the glory of God," (Romans 3:23). This includes Old Testament prophets. Noah is noted for getting drunk (which is also considered a sin in Islam). Abraham slept with a slave girl, even in spite of being credited with believing God's promise of a child who would be born to Sarah. David committed adultery with Bathsheba, then killed her husband and took her as his wife.
Now, in spite of the claims of modern Islamic apologists, none of the sins of the Old Testament prophets include pedophilia. In particular, it is claimed that Rebekah was only 3 years old when Isaac married her. This, however, is not consistent with the fact the scriptures call her a "young woman" (Genesis 24:16), and describe her carrying a pitcher to draw water from a well, being quite capable not only of gathering water for herself, but also for Abraham's servant and all his camels. Furthermore, according to the Bible, the "age for love" is when a woman is mature, and in particular she at the very least has fully formed breasts (Ezekiel 16:7-8). Thus, no prophet, nor even prominent figure, is noted to have married a child.
By contrast, Muhammad married a girl named Aisha, taking special care to note that she was 7 years old at the time of their marriage, and 9 when he, at more than 50 years old, consummated that marriage. The Islamic sources even note that Aisha would still play with dolls when he came for her, which was impermissible for girls who have started puberty. Thus, Aisha was, in no way, ready for marriage, much less sex with a 50 year old.
We see, then, that there is no comparison between Muhammad and the prophets. To be sure, the prophets did sin, and some of those sins, such as polygamy, they did share with Muhammad. But Muhammad's sins, first of all, were far worse. No Old Testament prophet included children in their sex lives. Worse than that, Muhammad repeated the sins of the Old Testament prophets after God revealed against them. Polygamy was permissible in the Old Testament because of the hardness of the Jewish community's hearts, but in the New Testament, it is revealed as being opposed to God's design, and is even sufficient to bar a man from having a prominent role in the Church. Of course, a Muslim could argue that Muhammad also had a hard heart, but 1. it doesn't work like that, you can't just disobey God's current revelations because you're stubborn, and 2. that's rather the point. God calls His people to a higher standard, not a lower one.
But ultimately, the question isn't "was Muhammad a sinner?" After all, even Jesus' own Apostles were sinners. Thus, Paul testifies "And I thank Christ Jesus our Lord who has enabled me, because He counted me faithful, putting me into the ministry, although I was formerly a blasphemer, a persecutor, and an insolent man; but I obtained mercy because I did it ignorantly in unbelief. And the grace of our Lord was exceedingly abundant, with faith and love which are in Christ Jesus. This is a faithful saying and worthy of all acceptance, that Christ Jesus came into the world to save sinners, of whom I am chief. However, for this reason I obtained mercy, that in me first Jesus Christ might show all longsuffering, as a pattern to those who are going to believe on Him for everlasting life. Now to the King eternal, immortal, invisible, to God who alone is wise, be honor and glory forever and ever. Amen." (1 Timothy 1:12-17).
But unlike Paul, Muhammad never repented. Being the chief of sinners, Muhammad continued in his sin, and ironically, it was his sin that got him killed in the end. His blasphemies continued until the time he cried "O `Aisha! I still feel the pain caused by the food I ate at Khaibar, and at this time, I feel as if my aorta is being cut from that poison." (Sahih al-Bukhari Vol. 5, Book 59, Hadith 713).
Chief among his blasphemies is his claim that Jesus is not even the Son of God. But here is what John, a true Apostle of God, says: "Who is a liar but he who denies that Jesus is the Christ? He is antichrist who denies the Father and the Son. Whoever denies the Son does not have the Father either; he who acknowledges the Son has the Father also." (1 John 2:22-23).
In denying the Son until the day he died, Muhammad dug himself an eternal grave, and will one day be cast into the lake of fire that is the second death, where he will be punished for all that he has done. But as Paul testifies, "This is a faithful saying and worthy of all acceptance, that Christ Jesus came into the world to save sinners, of whom I am chief." Being himself a pattern to those who are going to believe, Paul preached the way of salvation. Muslims do not have to join Muhammad in Hell. Rather, they can repent, and receive the gift of eternal life by confessing Jesus as Lord, and believing in their hearts God raised Him from the dead.