Most social networks have limits in place for the number of people you can "friend" or "follow". For example, Google+ currently has a limit of 5,000 people you can have in your circles, and you have a limit of 5,000 "friends" on Facebook. This is to prevent people, companies or automated programs from abusing the system. On Twitter, it was possible to mass follow thousands and thousands of people in the hope that they would follow you back. This has caused many Twitter accounts to abuse this and follow tens of thousands of people without creating engaging content and without many people following them. Not good.
Twitter was launched in March 2006. But it wasn't until July/August 2008 that Twitter started placing limits on the number of people you can follow. Twitter had huge spam problems (as Twitter co-founder Biz Stone was blogging at the time). There was a lot of backlash at the employee data time and a lot of confusion (see this ZDNet post on Twitter Spam Efforts Going Too Far and this blog post by Brad Williams on Twitter Follow Limits Are Jacked).It's commonly believed that Twitter has introduced a 2,000 follower limit, which means you can only follow 2,000 people. However, this is not strictly true.
The next report According to author, Rob Brown, the magic number is actually 1819! This means that if you have 1,819 followers, you can exceed the 2,000 limit. Below 1,819, you are limited to 2,000 following people. Above 1819 you can follow this number +10% (or 182). At 1,819 + 10% that means you can follow 2,001 people and you just broke the 2,000 limit - hooray! This (as Rob Brown mentions onThis means that if you have 1,819 followers, you can exceed the 2,000 limit. Below 1,819, you are limited to 2,000 following people.