I've had up to 6 in a 75Gal, a couple have been lost due to certain catasrophes or cataclysmic events. (maracyn2 cloud caused one to stress right out, found it a week later stuck in a castle.) the other had been injured and it just got worse.
You can keep them in a 20 gal
for a couple of months, I had 2 in a 20 gal
for a month, they are low polluters, but need the space to race.
They are a very beautiful fish, but don't plan on any neons or glowlight sized tetras, they become snack food.
Try to keep a 2-1 ration of femals to males, of the 4 I have left, all 4 are male and they fight a lot now since they are maturing.
getting them all at the same time would be advisable, too much an age difference may cause other problems (mature males terrorizing immature females.)
Included an old picture I took of 5 of them together. They could clear the length of the tank in less then 1 second and that's 4'
Also, how big are they at the store right now? if they are only an inch or less like the ones I've seen locally then you'ld probably be fine, just don't overfeed or they will get too large for the tank.
120 Gal SW, 150lbs Prem. Fiji, 23Gal Sump, 2x250w 15K MH, 4x65w Actinic
3 x Halichoeres Chrysus (1 surfed), 3 x Nemateleotris magnifica, 1 x Centropyge bicolor, 11 x Scarlet Hermits, 6 x Zebra Hermits, 40 x Astrea Snails, 6 x Nassarius Snails, 3 x Cerith Snails