Depends on the tank mates.
I would suggest corydoras as bottom feeders. These should be kept in groups, so around four pygmy or panda cories on the bottom.
If you want just neon tetras on top, then you could get away with around 8-10 with the four aforementioned corydoras, provided that you maintain the tank carefully and that it is fully cycled and that you only add a few fish at a time (so as not to add more nitrogenous wastes than your beneficial bacteria can handle).