I think the number of rats depends on both you and your current rats. If you have the room and time of day to give each individual rat attention and think you can handle more, then the only other limiting factor (besides money) is your current rats. Would adding more rats make your current rats happier or would it do just the opposite, because it takes away from the time you're spending with them? How does more rats affect what you have now in terms of time spent with you and over all happiness and health? More rats means more playmates, but it also means more stress.
For me, I was very happy with my two boys, but part of me really wanted to add a third (and a thirty-sixth, haha) to the pack. It took a lot of debating with myself before deciding to go with it, and I only did because I have the room in the cage with the boys, and because my boys' personalities collide so much that I figured Isaac needs a buffer from his hyper brother, and Styles needs a playmate, hence bringing the new baby into the pack.
A lot of people are right in thinking about money and their own time, but often times the current rats are forgotten in the process, does a new playmate benefit them? While rats are social creatures, just like us humans, they are all individuals and not might appreciate even more playmates, just as some humans prefer only a small number of close friends.
Not sure if that makes sense...but there's my two cents!