I know you're not allowed to cast more than one spell on an opponent's turn. Does this include Counterspell?

If it's the Wizard's turn and he casts Acquisition on me, and I counter it with Counterspell, then if he casts Mesmerism on me, can I counter that too?

I'm assuming no!

Correct. You could use 2x Counterspell if it were your turn as Player A and had 2 Counterspells in hand, if player B and C both tried to cast spell on you.