Your tank holds roughly 34 US Gallons. Typically, the advice would be 1" per gallon maximum, in your case, your surface area is a little more then a regular rectangular tank, so 34" of fish is a good figure.
Barbus arulius grows to about 4"/100mm in a tank. Nice fish, easy to keep, eats just about anything, flakes, frozen bloodworm, etc., a general varied diet. They do like crushed peas!