After you've left-clicked to accept the radius, the bevel-tool is still active and you can 

  - press <kbd>numpad</kbd> <kbd>+</kbd> / <kbd>-</kbd> to change the segment count. If you don't have a proper numpad: in _Preferences_, go to _Input_ and check _Emulate Numpad_
  - or.. press <kbd>F6</kbd> and adjust segments from there:

 [![enter image description here][1]][1]


  [1]: https://i.sstatic.net/yo0WJ.gif