What do you think is the max amount of instagram accounts without a proxy?
I'm currently using 3, but i know agencies (who dont use bots) to use up to 20 without any issues.
Any ideas when it'll become suspicious?
It depends on what are you going to do. If you are botting and doing Black stuff, you MUST go for 1 account per proxy, because if not, when they ban you one (they will) all of them are OUT.
But if you are going FULL white I don't know if there's really a limit, for example I have more than 15 accounts on various phones with my WiFi and there's zero problems. And when someone uses my WiFi, friends, family with their accounts probably IG knows that in my IP there's more than 20 accounts.