There are no hard and fast rules on Twitter about who you must and must not follow, but there are some norms about when you should and should not follow people. First, remember that you can't and shouldn't try to follow everyone. There are thousands of tweets sent every second, so there's no point in trying to read every interesting tweet out there. Additionally, who you follow on Twitter is visible to everyone who looks at your profile. If you follow thousands and thousands of people, it will be clear to your profile visitors that you are following them for the sake of following them, and that you're not really interested in the tweets of people you're following. There is also additional monitoring of your account that happens once you follow 2000 users. Another thing to remember is that it's generally considered polite to follow someone back if they follow you. This is particularly true if you know the person in real life, or have conversed with them on Twitter or other websites. Keep in mind that if you don't follow someone in return, it can seem like a snub. Additionally, they can't send you private messages, so they won't be able to contact you directly. You can always unfollow them later if you need to.