Legal Question in Business Law in California

There is no legal maximum number.

As many as are willing to sign up.

Generally there are no "people" involved in a joint venture. If it is an organization of people it is a partnership, LLC, or corporation. A joint venture is a joining of business entities together for a business purpose. Usually it is for a specific project of some kind, and/or a limited time, because if the two businesses want to join indefinitely for all purposes, they are better off just merging or one buying the other company. A joint venture is also not really a defined legal entity. J.V.s can be partnerships, LLCs or corporations themselves. For example, many major construction projects in California have been built by joint ventures between Obayashi Construction Corp. and Webcor Builders Corp., both corporations, under joint ventures that have been limited partnerships or LLCs. Eventually, they were doing so much work together, that the Obayashi parent company acquired Webcor.