The rule of thumb is typically one fish per 10 gallons. So you would need approx. 2 fish.
I think you meant 1" of fish per 5g of water. I'm not familiar with the 1 fish per 10 gallons rule..
919 meant a rule of thumb for cycling with fish - not for stocking. Kind of like the "1 cocktail shrimp per 30-40 gallons rule" for fishless cycling... but much more humane!