Instagram periodically take measures to identify and remove fake accounts from its platform. And if it identify the bot followers, it will also remove them. If you notice a sudden drop in your follower fount, it could be a result of Instagram's efforts to remove your followers.

Manually:- Go to your profile, tap Followers, tap the three dots next to the follower you want to remove, and tap Remove Follower. Using a third-party app:- Give the app access to your Instagram account, and it will scan your followers and identify the ones you're not following back.

One of the 5 most used ways to find out if someone has fake followers is to check through Social Blade. All you have to do is create an account, select the platform and put the username of the account your want to check and see the statics and growth.