Check out this site. They have a bunch of pre-built computers if you don’t want to build your own. You can also pick a basic build and upgrade one or more components.
https://www.ibuypower.com/Site/Computer/IntelDesktop
(this is the intel processor section)
If you want at least 8 cores in an intel processor, I believe you’re looking at at least a high end i7, or an i9 processor (which all have at least 8 cores iirc). I believe all i5 processors are 4 cores. AMD processors have more given codes at each price point but I’ve read intel offers more general computing performance (I’m not knowledgeable enough to really comment on the differences).