A recent study conducted by the University of Southern California estimated that between 9% and 15% of the active accounts on Twitter are run by robots and not humans.
According to Twitter’s own website, they have 313 million active users on Twitter – which puts the number of possible bots on Twitter close to 47 million (46.95 to be exact).
The study used more than a thousand features which were pulled from public data and meta-data about users: friends, tweet content and sentiment, network patterns, and activity time series.
As some have pointed out, Twitter bots may have negative connotations, but some bots can be extremely useful – like to alert people of natural disasters or to use for customer service instances. On the other side, the bots can be used to do a lot of bad – like spread terrorist propaganda.