We recommend that you follow less than 100 people per day, particularly when you have less than 2,000 followers. Larger accounts can often find that they can follow more people without a problem, however Twitter has a hard limit of 1,000 follows per day. Once you follow 1,000 people in a day, no matter how large your account is, additional follows in the same 24 hours period will be rejected by Twitter.
There is no limit on unfollowing, Twitter does not ban people for performing large number of unfollows in a day. The only caveat to that is that we recommend you do not unfollow then refollow the same people repeatedly as Twitter considers this to be aggressive activity and may result in your account being banned.