There is no set amount of followers that will determine success for a Twitter account. As stated above, there are factors that contribute to whether a Twitter account is a successful one or not and one of the bigger determining factors is whether on not your followers are real.
There are so many fake or junk followers on there that are not real and those will basically contribute nothing to you other than just a number added to your follower count.
I would worry more about the quality of your followers than the actual number of followers.