That is a good question that I have asked others myself. The only reasonable answer I have gotten is those you follow cannot be double than your followers, but I have seen twitter accounts with a higher ratio than that. Try emailing twitter and ask them.
I heard on the grapevine that if you have less than 2000 followers you can not follow more than 2000 people yourself. Can anyone confirm that rumour?
Actually there is no particular limit or ratio but to maintain or keep the credibility of your a/c it is better to minimize the ratio
Your followings won't grow after the followings reached 2000 while the followers less than 2000, you have to increase the number of followers first. After you got 2000 followers, then you can add 10% followings for your accounts. For example 2000 follower, 2200 followings;