If Thunderstaff is untapped, it prevents 1 damage from each creature, each time it would deal combat damage to you. So if three creatures with double strike attack you and aren’t blocked, Thunderstaff will prevent a total of 6 damage.
As long as this artifact is untapped, if a creature would deal combat damage to you, prevent 1 of that damage.
, : Attacking creatures get +1/+0 until end of turn.
, 
: Attacking creatures get +1/+0 until end of turn.