Okay so this is going to be a complex answer (as others have said). for some computers or cases, one fan can be enough, for others? No fans! (Linus Tech Tips did a video about a computer that was essentially a giant vapor chamber).
Personally? I have a Corsair 780T with a biometric fuckton of fans (4 intake, 3 exhaust)
(Google Images “Corsair 780T”) in mine, there’s two 120mm fans on the bottom by the power supply. The rest are 140mm.
My work PC? 3 fans! Two 140mm ML140’s and a single ML120 (all three Corsair)
(Google Images “Define Nano S”) This pic actually works out quite well…. Replace the Noctua CPU cooler with the stock Intel CPU cooler and change out the GPU for an Asus Turbo 1060 (same blower-style fan). Current thermals as I sit here with 24 chrome tabs between two windows on two 1080p panels as well as Steam and Discord active in the background? around 40C.
My “mobile” rig? Single fan. Yup, one single big-ass fan. 200-mm in the front….. Okay technically a blower-style fan on the back…. due to the H5 SF, but still. Temps on all of them are just fine.
This really depends on your case, and the kind of airflow setup you’re going for, whether or not you have a radiator for a watercooling system, etc. Usually, you want to mount as many fans as you have space for in the front for intake. Exhaust, one in the back is sufficient in most cases, though if you experience heat issues you can add more on top. Some cases also support side fans which are usually better to have as intake than exhaust in my experience.
In the end, different cases are designed differently, and it’s the case design that will effect your thermal performance more than anything else (except maybe your cable management if they’re just strewn all over the case). You just want to make sure you pick a case that has good airflow, and then make sure the cool air’s path is relatively unobstructed as it moves through the case.
It is also important to note that dedicated coolers on your CPU and GPU effect your temperatures far more than ambient case temperature does, so it’s always good to make sure you get decent cooling solutions for the CPU, and pick GPUs with good coolers as well.