Does this mean christianity is a polytheist religion?

It's clearly polytheistic, and Christians have to do some serious play with semantics to even try to say otherwise. The "Triune God" has to be somehow imagined as three who are one, though they interact with each other in the Bible, and as you mention, there's Satan, as well. In addition to eternals such as angels and demons. And among the Catholics, even more, as there are saints and the Virgin Mary who are worshiped... though again, they'll call it something else.