Hi underwater_mermaid_
Since your tank holds about 18 US gallons, you could keep that many fish in there, and a few more too.
But I suggest that you reconsider having that many of them gouramis.
First, corys are schooling fish and need to be kept in groups of 3 or more, so you pretty much need to get more than one. From their point of view, it is a case of the more the merrier.
Corys are friendly and love having companions of their own kind, but this is not the case with the gouramis. Gouramis are territorial and tend to defend the area that they claim for their own. With 4 gouramis, this could well turn out to be a problem. I would suggest that you get no more than 2. The honey gouramis are usually thought of as being more peaceful than the dwarfs as well.
Since having fewer gouramis will give you more space to fill, you could then increase the number of neon tetras which will give you a much nicer display too.