I follow 1000 generally every day, which brings me about 2400 followers a month. This is for the few month or so. Then the follower count per month goes up as the twitter account gets larger. I have never experienced in my 8 or so years of twitter any bans, blocks, limits, throttles or any issues relating to what I do. The worst that happens is the account gets locked, and i verify and unlock it... sometimes I have to change my password.
I'm currently managing 12 accounts, all follow 1000 a day most days.