An air-to-surface missile is a missile which is fired from an aircraft to the ground.

sam

Trishul is a surface to air missile . Trishul has a range of 9 km and is designed to counter a low level attack with a very quick reaction time. It is all all-weather surface-to-air-missile.

sam

Surface to air is shot from the ground into the air at the target. Air to air is fired from another aircraft. Actually there is very little difference. just where the missile is fired from.
Of course, there can be many more surface-to-air missiles (SAM) than air-to-air (AAM) because an enemy aircraft can only carry a limited number.