A scoop of tea leaves

This would  depend on the strength of tea desired.
A medium strength tea requires a 60 ml teapot
to be filled with 1/2 its capacity with tea leaves.
For something a little stronger, fill the pot up to 2/3 capacity of leaves.
A mug with the capacity of 8-12 oz of liquid, 1 teaspoon of leaves will suffice.
For an English style of brewing using a teapot, use 1 teaspoon per person and 1 teaspoon for the teapot.