I see 2903 sets on "Select Game" window (2888 total sets).
You can see this info just below "ROM Dirs" and "Scan ROMs" buttons.
In order to see Megadrive only, just unpick all the other boards in "Filters" box; in my case, doing that I'm seeing 565 sets (instead of 2903).
That should mean there are 565 Megadrive sets.