The only way to delete someone who is following you is to block them. (Go to their profile and click "block") If you want to delete someone that you are following then go to their profile and click "unfollow".
Each user has a different amount of followers - there's no set number. In Twitter, you can have followers as many as you want. There's is not specific number of followers a certain twitter user can have. The more the better.
When you follow some on Twitter you're automatically added to their followers list. By going to their Twitter page and clicking on "followers" your username will appear among the followers with "that's you".