Honestly, it will depend on your capabilities to thoroughly grasp and understand every theme. Definitely in an essay, 3 or 4 themes will do. Saying that though, it's a good idea to have excess, just in case some of the themes you prepared don't address the question itself. I would go for maybe 6-7, but if you feel comfortable with 10, then there's nothing stopping you from doing so.
I should also say it depends on the text itself as well. For example, I noticed that 1984 (my Common Module text last year) had more themes to think about and discuss than King Henry IV (my Module B text), so it gave me more avenues to explore in terms of themes.