If you are below 2k followers then up to 1000 a day (may vary a bit, like 900-1000). Basically, you can create 1k followers in 2 days, if 50% of people will follow you back After 2000, there is a limit, not more than ~200 difference between followers and who you follow. For example, if you have 2500 followers, you can follow till 2700 in that day. Even if you have 2k for example, (though this is not very precise too, may vary on +50-100, not minus though).
I don't know if there is a limit, but here is a suggestion to get some more followers. Right some posts into some popular streams such as #tech or whatever you are twittering about and you usually get a lot of followers.
I think it changes when you get into the thousands though as I see people say 55k following and 53k followers. I think after 2000 it might actually be a 10% limit.
After your reach 2000 following, you can only follow 10% more than the number of followers. As for per day, Twitter says the limit is 1000 a day for people you can follow. However, you shouldnt do that many. It could draw too much attention from Twitter and could get your suspended. From my experience, I'd limit it to 250 per day at most. But if you do try more than that, do it with an account you don't mind losing if it gets suspended.