What could I add to my tank? 10 gallons?

I have 2 platys, a snail, and some ghost shrimp. I used to have a lot more fish, but realized I fell prey to a common first time fishkeepers mistake. Overstocking! So the chemistry in the tank easily fell out-o-wack and fish suffered. But I've learned and am ready to make the correct move. So I was planning on adding 1 more platy. Because I know they much prefer groups of 3. Could I possibly add 2 more? I want the tank to be PERFECTLY stocked. Not to under. But not close to over stocking or fully stocked. What would work? I do clean the tank once a week with a gravel siphon!