Hola Annie

It's true that the noun in the first sentence is "novia", singular, because we are saying that Miguel has one girlfriend, but the second sentence is now using the plural to refer to this same word, as you see in the English "you have too many". In this case we need to express it in the plural as in saying "you have too many [girlfriends]": "tú tienes demasiadas [novias]"

I hope it clarifies it.

Saludos

Inma