I read in a marketing on Twitter guide you could follow about 300 per day and be safe. my friend was followed 400 a day and had no problems.
There isn't a set number, but keep in mind the ratio. They won't let you follow more than 2,000 if you don't have a decent amount of followers yourself.
As jwmann2 says once you get to 2,000 you will have to have a similar amount of followers, Just take it easy and follow 50-75 a day. Just make sure you contribute something or all you will just get ignored!
Twitter has changed. In the past i had accoutns banned for over following. Now it has set limited. I followed over 900 until twitter will say i met my following limit for the day. Then I go back the next day and follow again. After much time doing this, twitter has not banend me.
twitter does not allow you to follow peoples rapidly, if you try to go against it then you will surely get banned by twitter and the reason will be "AGRESSIVE FOLLOWING"