There is no hard and fast rule about how many affiliate companies you can use. I used two every month, CJ and Neverblue, but reality I've signed up and made money with about 15 different companies. Most of these companies will let you stay with them, even if you're not making any money. I like having to deal with more than one company, because the rates you get on the same offer can very from company to company.
I wouldn't worry about being loyal to any one company, just worry about doing offers you can convert, getting traffic to those offers, and watching a rate of return. There is one offer that I flip back and forth on every month that's run by two separate companies. When the CTR gets too low, I switch to the other vendor. Maybe it's just me being paranoid, but I have a pretty good idea about how my traffic converts, so when I see something not converting after a day or so, I take that as a sign to go. Just my two cents.