You could get away with ten, it seems you have enough knowledge about aquariums that a little bit of overstock wouldn't be something you couldn't handle. Tetras are pretty picky, but platies can be worse. Both are semi-aggressive. Farmed tetras are not hardy at all, they really do best in soft water/low ph, but in a healthy tank ph isn't all that important. Hopefully you'll have good luck and healthy fish.