How many would you like to keep ?
30 to 40 of them would not be out of the question, particularly in a 20 long, so long as you have good filtration, and you're prepared to do say, weekly wcs, along with maintaining good tank cleanliness.
Its more down to you and your willingness to do the work involved taking care of X number of tetras. You could play it safe (easier) with only a dozen, or go big with 40, but then have work (water changes, filter cleaning, etc.).
40 neons in long tank would be a sight to see... But, lots of work.
i would say do about 17. yes its an odd number but for schools odd numbers look better. i have a 20 with an angel and its really not a lot of back and forth swimming cause its not really long and if you get to many they wont really get to do good laps. you can do other tetras or something else and have it well planted also