Hey i saw a youtube video on how to optimize your pc. And he went into msconfig and into advanced options and he selected all the cores. I was wondering if i should do that or if i should leave it a 1.
No. You should not change those settings.
That is not optimization. It puts limitations into bootloader. You use it only, if there is some issue.
So far I haven't seen any situation, where it would make sense to use it. May be only if you wanted to simulate, how your pc would perform with less cores available.