well if 30+ full grown mixed pygos (reds, caribe, terns, piraya) can shoal well in a 5x2x2.5 tank then i reckon ten would be a good number in a 42x18x22
best way to find out is just do it. but dont buy them large, buy them at about 2" and grow them all together, they may look slightly cramped but that is how they swim in a proper shoal in the wild
at night when they sleep, as long as they arent all crammed in lol, its fine they should all find their own little space to chill
in that 5x2x2.5 tank they would go 4 deep and swim up and down the tank real close knit, inspiration
it was like watching them in those underwater clips you see on the tele