I cant play multiplayer after a few hd formats. It says serial key has expired, because ive used it too many times.
This sucks, any game that requires a serial number, i will never buy again.

Anyway what can i do about this for tw2?

Your best bet is to contact support and explain the situation, most likely they'll be able to send you a new key.

And I'm very surprised to see there are games on Gog with limited activations, even if it's just for multiplayer. That's the kind of thing that really shouldn't happen here.