I know like dragonfish said their bioload is very minimal, I think a dozen or so will be fine.
Cardinal tetras are very similar, much more colorful then the neons IMO
, They are a little larger and school tighter.
Danios are another smaller fish but they do poo alot. Even some guppies would be neat in there, maybe 3-6.
Its a 7.5g tank by the way, id say a dozen neons with some plants.