Paul, in context, leaves no doubt about his monotheism.
1 Cor 8 deals with food offered to idols. As such, we expect the text to deal with "so-called gods". Moreover, I find it interesting that you chose not to include v4 in your quote:
4 Therefore concerning the eating of things offered to idols, we know that an idol is nothing in the world, and that there is no other God but one.
5 For even if there are so-called gods, whether in heaven or on earth (as there are many gods and many lords),
6 yet for us there is one God, the Father, of whom are all things, and we for Him; and one Lord Jesus Christ, through whom are all things, and through whom we live.
7 However, there is not in everyone that knowledge...
After having emphatically stated that their is only one God, Paul goes on to talking about "so-called gods" (calling them "nothing" in v4), saying that these so-called gods comprise "many gods and many lords." Moreover, v7 refers to "our" understanding of God as "knowledge" - Paul and his companions know that "there is one God, the Father, of whom are all things." To others it might seem that there are many gods and many lords, but Paul and his companions know otherwise.
If we take v5-6, out of context, we might conclude that they teach henotheism. Alternatively, we could simply take the clause "there are many gods" in v5 out of context and say that the Bible teaches polytheism. But in order to understand what the author meant, we must leave his words in the original context.