From many answers , I figured out that aggressive behavior in following, unfollwoing, tweeting, messaging..etc is the reason beyond most bans and not how many accounts on each proxy.
This means, if I am to be aggressive in using these accounts, I better use 1-2 per proxy ,so in case this IP is blacklisted I don't lose any accounts.
There is also another thing; where from do we get these proxies? There are providers who sell one dedicated private proxy for $ 3 and another sells the same for $1. This can't be real. Many think that there are sellers who claim selling private proxies for $1 while they sell you a shared proxy. This means this proxy is sold to 2-3 people at the same time and if one of them is doing something wrong with it and get banned, all will also do if that ban is in the same social medic platform.
Thus, the most important thing is to find an honest seller before deciding how many accounts to set per proxy.