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Each Twitter account has its own Twitter id. You can easily find your own Twitter id by looking at the URL for the RSS feed for your Twitter account. In the early days of Twitter, Twitter id's were sequentially attributed. This made it easy to track the number of total Twitter accounts created (not necessary the number of active Twitter accounts). Jason Kottke published almost a year ago an article based on such stats.
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Because of the growing popularity of Twitter, the Twitter developers decided to change the policy of generating Twitter id's by the end of November 2006. From that moment on there were gaps between the id's actually used. Assuming that the Twitter id of a new created Twitter account is equal the total number of Twitter accounts does not seem correct to me. The exact number of Twitter accounts created until now (and the number of active Twitter accounts) is only known by the people at Twitter Inc. And they haven't disclosed it until now ...
By the way, currently it is not possible to register Twitter names containing spaces. In the early days it was possible to register such Twitter names. Some of these names are still in use, e.g. twitter.com/there you go.