I think your host could easily do one chocolate and one vanilla.
In my experience, more people like chocolate than vanilla (usually the chocolate goes faster). If I had to guess, I would say it is usually 2 to 1 chocolate to vanilla.
I think you could ask for cream cheese filling, if it is what you like. I think vanilla with a fruit filling (raspberry) is also popular. But it is your shower and if your host is asking your opinion, then you can ask for what you like.
I don't know how many people are expected. Not all will want cake. It will also depend on what else your host is serving.