Welcome!
Birdo is correct, the guideline for
small fish is 1" of ADULT fish to 1-2 gallons. I prefer to give more space than less, unless you don't mind doing more frequent water changes (I have a slightly overstocked tank I just do more maintenance on: 2-3 water changes a week instead of 1, it's really not that big a deal.)
For larger fish, you need to allow far, far more space per inch of fish: for example, a 14" Oscar is NOT going to be happy in 14-28 gallons of water!!
Your tetras will get to be appx 1.5" at maturity, so they need 1.5-2.5 gallons each (I'd go with two.)
I really don't know anything about rainbow sharks or the loaches, but I am 90% sure that you are already overstocked on adult fish size in that tank. I'll let someone that knows about those kinds of fish address that part. -_-"
Tetras need to live in groups of 5 of their own kind or more: 6-12 is the ideal number.
Also remember to never add more than 3-4 fish to your tank at a time (especially in a smaller tank), because adding too many at once gives a sudden spike to your bio-load and will throw your tank into a mini-cycle.
Hope this helps & good luck stocking.
