I have no reason to believe that there is a maximum. However, CJ can kick you if you are not making sales, and if you sign up with 600 advertisers and don't make a sale in 6 months it won't help you! If you are making sales there's no reason for CJ to impose a maximum that I know of.
You can join as many as you feel fit, just remember that some companies if you are inactive for a bit will terminate you for not making money. So make a list of what is important to you and do those niches first, then slowly add more, it can be over whelming and you might want to sign up for as many as possible when you see who is on CJ, but keep a cool head