Hmmm I think you need as many as your tank will allow, the more you have the better they look. I have 10 and they are not always together although usually at least 6 of them are. I also have 6 black neons, they are a bit larger and have a different body shape but have the same effect.
I have found gouramis are a good mix with neons, so far no problem at all and of course some corys for the bottom.