Whilst your first instinct may be to join as many affiliate programs as possible, I think it’s better to keep things to a manageable number.
Different people will have different views as to how big or small that number should be but I think it should definitely be in single digits for any given niche that you’re involved in.
For instance, in the self help niche that I like, I’m in three main programs and two of those generate the lion’s share of the commissions i earn there.