Instagram have not made an official statement related to the limit of follow unit. You can follow the maximum users on your choice but Instagram has set parameter that you cannot follow more users in a day. If you do it, your account will be the part of the spam. Instagram can block your account temporarily. To avoid the spam, they can permanently block your account.
Always try to follow maximum 5-10 users at a time. If you are genuine user, then your limit will be less than the 10 but you can follow more. Instagram will not harm your Instagram account.