additional fans needed?

I am designing a small case for a pc with a mini-ITX motherboard without GPU. I am wondering if it is possible to get away with a CPU cooler (Noctua L9i for example if the case with vents is right above it? Or is another fan needed, on top of the CPU fan, or can it be placed besides the CPU fan (to keep the case as flat as possible)?

thanks for your help

It depends on the CPU used, the application, and the exact fan configuration and case you are using. After we have all that info we could guess, but you won't know until you build it and start testing.

Option 1:
Thinking i7 4790K and the case would be just over 18 by 18 by as little height as fits (probably around the 5cm mark). I would have perforations on the sides and a lot on the top, especially right above the cpu cooling fan.
There are two alternatives if that isnt enough:
Option 2:
Keeping the height as low as in option 1, but adding a 40 or 50mm fan if i can squeeze it in in the top of the case next to tge CPU cooling fan

Option 3:
Raising the top of the case by just over 10mm to fit one or more slim 10mm heigh fans in there (is right on top of the CPU cooling fan the best spot?)

I am hoping option 1 is enough but dont have abny experience. Hoping someone can give me an indication before i build my first prototype

The issue with using no case fans and such a powerful cpu (overclocked, I assume) is that your cpu cooler will start to recycle the same warm air for cooling. What I would do is make a duct so that intake air is 100% from the outside of the case. With that, the warm air will leave through whatever holes your case will have, but adding an exhaust fan would allow you to dump the warm air outside your case in a location where it would have less chance of mixing with the intake air. If you want the case as flat as possible, adding two small exhaust fans in the back (60mm, or a centrifugal fan) would allow your cpu to get cool air through the top and exhaust the warm air through the back.