yeah for an 11 gallon I would say only put in AS. Depending on just how bad your infestation is will depend on how many AS you want. 6 might be a good starting point if your infestation is bad, you'll see an immediate impact.
What else is in the tank? Can you cut down on feedings?
Lower your feeding. If you watch your feeding the snail population will slowly decline and you can feed the rest to your puffers. I almost ran out of snails to feed my puffers because I stop feeding my shrimp tanks. If you want one assassin snail does a good job without taking over the tank itself.