It will depend on the strength of the magnets as to if you use 2 or 3 really. You want it to be able to stand on the switch without firing under the weight of an normal atty (
kayfun etc) with an 18650 battery in it with locking the switch. I have found the ones sold by
fasttech in the 10 packs need 3 usually while the FatDaddyVapes are pretty much spot on with 2 magnets. I have a
fasttech 3 magnet installed in one of my 3 nemesis mods and FDV in the other two with all using his shorter throw bottom switch pin. I check the switch after a few days of use or before changing to one of the others when cleaning them.