I would recommend a modest school of seven cardinal tetras. If you wanted a larger school, I would suggest trying a smaller breed like blue or green neon tetras. You might be able to do 9 or 10 of these fish and they look very similar.
Cardinal tetras are small, slim-bodied fish so the Inch per Gallon rule of thumb works fairly well for estimating appropriate stocking levels. The important thing is to count the fish based on their expected ADULT size rather than the size at time of purchase, to avoid over-stocking the tank. Cardinal tetras can grow to about two inches in length, so a 14 gallon tank could safely house about seven of these fish, with no other occupants.
Ideally, you would also want to prepare the tank for these fish by establish the biological filter before adding livestock through fish-less cycling. This would minimize stress on the fish and allow a smooth transition from the pet store to your home. It just takes a little patience while you wait for the tank to settle in.