If the design of your case relies on 12cm fans, I would focus on finding your sweet spot, i.e. your optimal balance of the following three factors:
– performance (mostly how much air the fan can move, but there is a bit more nuance to it than raw power)
– silence/noise/vibration (this includes not only the dbA volume generated by the rotating blades, it also the behaviour of the bearing; also consider the ease of difficulty of applying additional lubrication the bearing in case you want or need to)
– price (my advice would be to go a bit beyond the best bang for the buck and go for the bang instead, as it's your ears and your comfort over many hours of work and play, so something like $5 theoretical saving per fan is just not worth it compared to more comfort; also, compare many different offers in detail, perhaps write down the most important parameters for comparison)
In some cases price can do the trick, as there exist very strong fans that are also very silent or at least very smooth (not an annoying sound, that is) — their 'downside' is that they can't really also be cheap. But $20 fans can be a whole different thing from $5 fans (although they aren't always; some fans are simply expensive; as a rule, however, there aren't really strong and silent fans that are also cheap, so you probably want to look at the higher brackets only).
From that point onward add more fans as needed/if needed, but do consider that sometimes more fans means more vibration and even more of the ordinary noise generated by moving air (such as when the trajectories of air streams collide against each other). But, as long as budget isn't a pressing issue, I would normally always be in favour of adding more fans as long as you can afford high-quality fans at the moment and won't be reducing your comfort in a noticeable degree. However, not at the cost of buying cheaper fans so you could buy 2 in lieu of 1, I wouldn't do that. I would buy 1 and wait for better times, then buy another.
I'll probably need to follow my own advice soon, as I need both a new computer and a new case for my old one anyway, and I think I'll look at those more expensive fans, $20 range, if I buy a case that doesn't have stock fans. Otherwise I'll look at those a bit later as replacements for the stock ones.
