Why can't I follow any more people?

If you find that you can't follow any more users then it is possible that you've hit the Twitter follow limit. Twitter restricts the number of people you can follow to 5000 if you don't have much followers youself. Once you reach this limit, Twitter won't let you follow anyone else until you increase the number your own followers. You'll need to allow some time to gain more followers and the restriction will be automatically lifted, allowing you to follow more people again.

More information about follow limits can be found on this page.