10 accounts on one phone is more than fine.
The IP that is associated with it is what can be an issue. If you are running heavy actions on all the accounts all day, then you will come across the "Account Compromised" message. And IG will treat your account as if it is botted, and give you a 7 day block.
with 5 you are safe, as long as you don't sequence the actions of the account. for ex. Follow 50 with acc1 then immediatly swtich to acc2 and follow 50, then acc 3 etc.... you need good delays if you are going to be using your home IP or switch mobile network via airplane mode switch, if you are using the phones 4g.