5 snails per m² can be a useful guide.
Trapdoor snails have gills & should be treated as fish.
Once you receive your snails, please fill the container they arrived in with water from your pond to acclimatise them to your water. Place the lid back on (which has air holes) and allow the snails to settle for up to a couple of hours before placing them in the pond. Any that float to the top have unfortunately perished and should be discarded.
These snails are a wonderful addition to any pond to assist with clearing algae.