The amount of servings per gallon of tea will depend on the size of the glasses or cups you're serving the tea in.
If you're serving the tea in 8 ounce size cups you would get 16 cups of tea out of a gallon of tea.
So you could serve 16 people 8 ounce cups of tea each out of a gallon of tea.
Now if you have larger glasses or cups then the amount of servings goes down but if it's 8 ounce cups you'd be able to serve 16 cups of tea per gallon of tea.
If you have 16 ounce glasses you're serving the tea in then you would only be able to serve 8 people with 8 glasses of tea in 16 ounce glasses.
The smaller the cup or glass of tea the more you can serve but the larger the cup or glasses of tea the less amount of tea you can serve.