My affiliate marketing site count is approaching 40 and will be at least 50 by the end of the year. Each one adds to the till each month, but some do tend to stagnate from neglect.
I agree that someone can probably only manage 4 or 5 and do it well, but quite frankly, I do not know which sites I would dump if I had to drop back to that level.
At this point, I just try to rotate through the sites and give each one at least some attention each month.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin