It always varies by case, ambient temperature, and your components, but in general, having a slightly positive pressure (more air being pushed in than being pushed out) is considered beneficial. It's super rough, but what I do is look at the rated air displacement (cubic foot per minute, CFM) of each fan (which is often given for their max speed) and try to roughly balance my case fans based on their CFMs.
So if you have 2 fans at default, one front 120mm as an intake and one rear 120mm as an exhaust, and the intake fan is rated at 60 CFM while the exhaust fan is rated at 50 CFM, that means you should have slightly positive pressure (more air pushing in to your PC than being exhausted).
In general, one intake and one exhaust is probably "good enough" for most builds, but adding more certainly doesn't hurt, and adding more often gives you the option of reducing the RPMs of each fan so that they're individaually quieter, but still pull/push more air overall (basically, more fans = they can run slower/at a lower RPM = they run quieter = their quantity compensates for their reduced individual RPM).
If you want to add one more intake and one more exhaust to the two fans that are already included, I think that should more than suffice. More is fine and probably would let you see minor temperature increases, but I do think there are some diminishing returns when you start getting into the 5+ case fan territory... though as mentioned at the beginning of the post, it's all case-by-case, so there's no one answer fits all.