"Are Christians polytheists?" I admit, I wasn't prepared for the question. I'm used to thinking of Christians as ardent monotheists.
Christians believe in one God only. However, Christians believe this one limitless God is a triune being -- that is, that He is Father, Son, and Holy Spirit simultaneously ... all coequal, all one.
Muslims may believe, based on passages in the Quran, that Christians worship three gods: God the Father, the Virgin Mary, and Jesus the Son. But this is based on confusion of the Biblical doctrine of the Trinity (Father, Son, and Holy Spirit as one God). This confusion is probably because it is difficult to comprehend how God could have a Son without also having a wife and, therefore, a physical union with her, and so have a repugnance for the idea. Here's one passage of the Quran (5:116) that expresses this (erroneous) impression of Christian beliefs:
"And when God shall say unto Jesus, at the last day, O Jesus, son of Mary, hast thou said unto men, Take me and my mother for two gods, beside God? He shall answer, Praise be unto thee! It is not for me to say that which I ought not." (Quran 5:116)
Frankly, many Christians would also treat that notion with distaste, because it is not what we believe at all. The following Bible passages express the Christian belief in only one God:
"Hear, O Israel! The LORD is our God, the LORD is one!" (Deuteronomy 6:4)
"Joseph, son of David, do not be afraid to take Mary as your wife; for that which has been conceived in her is of the Holy Spirit." (Matthew 1:20)
Jesus said: "I and the Father are one." (John 10:30)
"Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit." (Matthew 28:19)
The belief in God as one God and that only He should be worshipped is a core belief of biblical Christianity. That said, it is also fair to concede that certain denominations, as with Roman Catholicism or some of the Orthodox sects are highly iconic and sometimes pray to the Virgin Mary or to Christian saints or angels, and commonly refer to Mary as "the mother of God".
But this is an area of contention even within our own faith, with many Protestant denominations seeing such practices as bordering on idolatry, while those adhering to such practices denying that is the case. So are Christians polytheists? The bigger answer is no, but there are admittedly some sects that skirt the boundaries. Similar disagreements with similar reformations and sectarian breaks related to idol worship have occurred among the historically Indian faiths -- Jainism and Buddhism, for example.