Where can I find data on the number of single propeller aircraft currently in operation in the EU and US?
You can download the entire FAA aircraft registration database and analyze it however you want.

I have no idea about EASA, but because aircraft registration is delegated to member states you'll probably have to look at each one individually (e.g. the UK). I don't know if all EASA countries consider aircraft registration information to be public or not.

You could also look for a commercial data provider that sells the data you need, of course.