It depends on a number of factors, but the old school/general rule is an inch per gallon. so lets just ball park it and say your tetras are an inch long each and your Otos are 2 inches each you've got 50 inches of fish for 50 gallons of water.
I'd say you're pretty well stocked as is.
I would probably add a few angels and call it good (or other medium center piece fish)
Much depends on the amount & type of filtration provided, the weekly volume of water you are willing to change & the varieties of additional fish you wish to add.
The one inch per gallon rule has long been proven obsolete since the advent of improved filtration systems.