I don't think they could really permanently ban IPs. Rather I would say there's a temporary ban that lasts a day / a few days / a week etc. I couldn't say exactly because I only use open proxies (not paid) so I just move on to another proxy as needed.
I suspect that the IP bans come from suspicious activity on that IP and especially multiple accounts spamming away using the same IP (which can be a problem with open proxies scraped off public lists - you never know who else is using them) or people using that IP to do heavy scraping.
I would definitely say you would be setting yourself up for pain trying to create 200 accounts per day from the same 20 IPs. You would need a lot more IPs IMHO eg. Instead I would look into getting say 140, then using 20 on Monday, another 20 on Tuesday, another 20 on Wednesday etc in a cycle. I wouldn't want to speculate on the exact number you'd need or rotation frequency, but I really don't think that using only 20 IPs would be a good idea. Or, you could get 200 and then use IP per account per day. I'm not big on the idea of creating multiple accounts per IP per day either.
That's the big advantage of using open proxies - although there are other people using them, there are thousands of open peas out there to abuse and you're not stuck using the same IPs all the time.