There's no single right number — it depends on how many requests you're making, how strict the target site is, and how quickly individual proxies get flagged.
Start with request volume
As a rough starting point, estimate how many requests you plan to make per hour and divide by a conservative per-proxy request budget (often somewhere in the tens of requests per hour per IP for cautious scraping) to get a rough pool size.
Factor in the target's strictness
A site with minimal bot protection might tolerate dozens of requests per IP before anything happens, while an aggressively protected site might flag an IP after just a handful. Watching your actual block/CAPTCHA rate tells you more than any generic formula.
Build in redundancy
Because a meaningful share of any proxy pool — especially free ones — will be dead or blocked at any given time, plan for a larger pool than your bare minimum math suggests, so a bad batch of proxies doesn't stall the whole job.