As many as you can my friend. But Heres a list of ones you want to get that wont outgrow your plant. Outgrowing isnt bad but it doesnt look good haha.
Anubias Nana - Gets about 7 inches tall at most and can adapt to ANY water and lighting condition. Also a slow grower
Crypt- I love this plant. Small and bushy. Makes a good mid ground// corner piece. Petco's with the actual underwater plant set up (not the plants sold in tubes) sell sell them for really cheap. about 3 plants for 5 dollars.
Amazon Sword- Only use this as a background plant. Can get up to 14 inches but if your tank is low-tech (no Co2 or lower light) the plant will stay around 8-10 inches.
Riccia- Great great carpet plant. Super easy. Not too high of light, may need some fert tabs (those are about 8 dollars at petco) but its not necessary. Tie it to some rocks or on top of driftwood.
Add all those with good lookin rocks and REAL driftwood and you have yourself a tank. I have like 60 dollars worth of plants in my tank haha