It's less # or size of additional fish, more what type of fish.
Dottys and clowns are all territorial so if you want another fish that is active and swims everywhere, 1 more is likely the most you can have. If you get more docile ones, you can probably get 2 or 3 more
For example, potential additions can be:
1) pick one: coral beauty, 6 line wrass, pigmy cherub
Or
2) pick two: firefish, basslets like purple or royal gramma (might fight your dotty depending on your rockwork), watchman gobie, banggai,.
OR
3) Pick three: pick two above + one of: cleaner gobe, sandsifting goby.
This will push your tank to the max.