That number on the left is the total length of the Timeline. Right click on it to select Frames. The number on the right is the current Playhead position. This,too, can be changed to Frames by right clicking on it.
Do be aware that if your Timeline start position is not at zero this will be reflected in the Frames starting number. The pics shows my Edit page with Frames and Timecode selected. The Timeline Frame count starts at 180000 because my start point is 1 hour in from zero and is at 50fps. 60 mins at 50fps equals 180000 frames. The Frame count in the Preview window also follows the original Timecode starting point
For the Preview window the left hand number is the clip length, the right hand, where the playhead is.
If you select in and out points in either window, the left hand number will show you the length of that selection.
You cannot change the Timeline Ruler or Timecode above the Track Headers to Frames.