Same thing. Your site has only one worker script, and each request is therefore processed by at most one Worker. You can of course merge many independent pieces of logic into one script -- it's code, after all.

Enterprise customers are allowed to have multiple scripts mapped to different URL routes. This is mainly so that different teams owning different parts of the site don't step on each other. However, generally if you stuff your logic into one script rather than multiple, it will perform better, since that one script is more likely to be "hot" when needed.