I personally use 2 accs per proxy, but i'd say 5 is pretty safe too.
Although Twitter doesn't really state anything about ips, but it's in the Twitter rules, that
you can use a phone number to verify up to 10 accs. If we think about this for a sec, this basically says that we're allowed to have 10 accs tied to the same phone number, so we might as well use them under the same ip. I'm not saying that it would be 100% safe, but it appears to be allowed as per Twitter rules. It also depends on other things, settings or if there's any other way to link the accs together (e.g. a domain) other than the ip. If you keep the accounts separated, you can get away with relatively high proxy:account ratios.
An evergreen example for this is the Twitter accs of students on campuses or the Twitter accs of people who use public wifi connections in cafes etc., where hundreds of people use the same ips, yet their accs won't get banned for the simple reason, because those accounts aren't tied by anything else, but the ip. If you can successfully mimic this, you'll get away with higher proxy:account ratios.
In short: Using 5 accs on the same ip won't get you suspended in itself.