* May I have one of these cakes? = correct if you want the whole cake
* May I have a piece/slice of this cake?
* May I have a piece/slice of both of these cakes?
* May I have a piece/slice of these three cakes?
I'll have a (cup of) coffee. (In conversational American English, we tend to omitt "cup of".) ...or...
I'd like a coffee with cream, please.