Legal Question in Civil Rights Law in California

You may raise as a defense that you're not liable for whatever it is that he's claiming. You might have other immediate options, but you'd have to be more specific. There's a possibility that once the lawsuit is concluded in your favor, you might sue for malicious prosecution. But, first take care of the issues now before the court. If, in any way, it involves your property, you might tender the lawsuit to your insurance carrier to select a lawyer to defend you.

How much can I counter sue for ?

Nothing, unless you have a valid claim of some sort against the plaintiff, other than your defense costs in this one.

What you do have is the right to defend the case, and possibly seek your attorney fees incurred if you win, depending on the nature of the case.

IF, IF, you can get the case dismissed outright by a motion for dismissal on the basis of it having no legal foundation, then you might be able to sue for malicious prosecution and abuse of court proceedings. Just winning at trial does not count for that. Neither does any settlement.