Are there many masculine nouns with feminine endings?

Not many, but there are some masculine nouns that are used a lot:

мужчина, папа, дедушка

 Almost all of Russian male names have feminine endings in their short version:

Александр – Саша

Михаил –      Миша

Григорий –   Гриша

Николай –    Коля

 How are masculine nouns with feminine endings declined?

They are declined like feminine nouns according to their endings.

В комнате нет папы (Genitive) = Dad is not is not in the room.

Женщина разговаривает с мужчиной (Instrumental) = A woman is talking with a man.