First, it depends on the proxy, it can be a good, fresh proxy, or even residential and you could use up to 5 accounts with no problems.
Secondly it depends on how you use the accounts, will you promote the same site on all, add the same link and bio details or are they all different in different niches?
Third, depends on the $ you want to spend, you can be paranoid and only have 1 account per proxy if you afford it, even if you could use up to 3-5. or you can use more accounts per proxy if you can't afford more with a slight possibility that if you do a mistake on one all can go down.