How many Twitter users can be mentioned in a single tweet?

So glad you asked!

Legal characters for Twitter handles are case-insensitive alphanumerics (36) and underscore (1). So:

37 legal two-character names (e.g. "@a") = 74 characters

7 three-character names (e.g. "@aa") = 21 characters

43 spaces to join 44 handles = 43 characters

74 + 21 + 43 = 138

So, the answer is: 44!