Another Professor can correct me if I'm wrong on this.
The deck is private knowledge when you search (this just means your opponent doesn't know what's in your deck). Because of that, Eevee's attack is one where you can choose however many Pokemon that evolve from Eevee as you want (the max is three and they must be different types of course). That means, 3, 2, 1, or 0 Pokemon can be chosen.
dmaster out.