Question about Intel Core i5 6600K BOX

First of all, I'm not quite sure if this question is meant seriously or if it's a joke... I'll give it a try anyway.
The case ventilation only serves to ventilate the inside of your computer in order to dissipate heat that is generally generated (e.g. from the mainboard, RAM, hard disks, etc). There should be at least one fan that extracts the heat, but there should also be a fan that supplies cool air. There is no such thing as too many fans, just too few.
Components that generate a lot of heat definitely need their own cooling! These include, above all, CPUs and graphics cards.
Graphics cards usually already have a cooler on them, but the CPU does not.
Therefore, a very clear no, the case ventilation is not enough. You have to get a CPU cooler if you don't want your CPU to burn out and die.
(Ok, it probably won't burn out directly, there are protection mechanisms for that. Your computer would probably shut down within a few seconds due to overheating. But the extreme heat (even if only for a few seconds) plus the emergency shutdown is definitely not good for the CPU).

This is not only emphasised, but necessary, as the selected processor is supplied without a cooler. The choice of cooler depends on how much overclocking is to be done.