@Maggeren answer sums it up pretty much.
It a depends on the type of proxy you're going go to use, and the purpose of what you're going to use the accounts for(i.e. personal, child accs, CPS, aff marketing, etc).
Ideally, 1 account per proxy is what you generally want to shoot for, but if your budget doesn't allow it, then its..
ipv4 you can stretch it to 2-3. HomeIP, I've heard of people claiming to run 4-5 on them. And on ipv6, it's best to not even push your luck as those proxies are cheap enough as it is.
Keep in mind, you're leaving a giant footprint, and rhn the risk of getting all the accounts on the IP banned at the same time, that's why I would suggest you stick to 1:1.