Legal Question in Civil Litigation in California

There is no limit, but you have to be concerned about being held to be a vexatious litigant. See CODE OF CIVIL PROCEDURE SECTION 391-391.8

Unless they have been named a vexatious litigant, there are no limits on the number. The only limit is that you must bring all related claims against a defendant in one action, and if you have a case against multiple parties for the same claims you must bring one action against all of them in most situations (this gets into the law of "necessary parties" which is too complex for most lawyers to understand; so I'll just leave it at that).

No limit, unless you are found to be a vexatious litigant because of filing unfounded cases.