@GPounda 20 is a great size school of rainbows. You can mix it up if you like, however the effect you get if you stick with 1 species is spectacular. The uniformity of it all makes it complete, imo. Also if you ever decide to dip your toes into breeding, you won’t hybridization to worry about. This is highly frowned upon.
Boesmani are a great choice. For something stunning but a bit different for roughly same price tag you can check out Melanotaenia Trifasciata ‘Goyder River’, Chilatherina Alleni ‘Wapoga’, or Glossopelis Wanamensis. The Wanam’s are incredible, when fully matured the males sport long, ragged fins, it’s quite the eyeful.
If you approach rainbows with patience, you will be rewarded with daily spectacles and breeding rituals. I always feel gratitude when observing them.