To be it as simply as it can be, an IP is an identity on the internet, just like your name in real life, instagram doesn't want one identity to have multiple accounts, 1 account = 1 identity (except for coffees but the device changes and the instagram are connected slowly over time and are very conservative with their followings).
Proxy = IP = new identity = new account.
So basically if you want to handle multiple accounts (5 more) you have to buy multiple proxies for each account.
You can do 1 proxy for 2 accounts but I prefer to keep it safe and stay with 1.