The $5/mo gets you 10 million dynamic requests (static assets are not included in this limit, so often a single pageview will be 1 dynamic request) and that would be across the whole workers product for your account, no extra pricing for extra websites, domains, or anything else like you'd see in most "wordpress hosting"
I run all my personal sites and client sites (one of them for a fortune 500 company) in the $5/mo plan, and the only time I went over that was when a client got hammered with malicious requests (and it was like $100)
Disclaimer: I have no relationship to cloudflare, I'm just a happy customer