Games up to today dont really need more than 4 cores. A very few games takes advantage from having more than 4 cores.
But take the new 8700k. Now you do not need it today. But lets say 2-3 years from now, with Ryzen having all the cores also. Programs and games should be getting coded in sutch a way that they will make use of the extra cores.
But by then both Intel and AMD will have released new CPU`s that is better than the ones you can get today.
So yes an 6600k or an 7700k is more than enough today.