The additional shots do not consume additional ammunition.
At 120%, this mod grants a 100% chance to fire one additional round (2 rounds/shot) and a 20% chance to fire two additional rounds (3 rounds/shot).
Equipping this mod and Lethal Torrent simultaneously will provide 180% multishot, giving the sidearm a 100% chance to fire 1 additional round (2 rounds/shot) and an 80% chance to fire an additional round (3 rounds/shot).
The in-game UI will consider any possible multishot capabilities when showing final damage numbers, and does not represent single-projectile damage. This is untrue when it is installed on a shotgun (i.e. a Bronco), in which the multishot adds to the amount of pellets and therefore it will remain a fixed value.
According to the reward table for Defense missions from Update 10.3, this mod can be obtained from Infested Defense missions. This does not apply to the Orokin Derelict, due to it having a separate reward table.
Equipping this mod increases the status chance of a weapon indirectly, but it is also calculated in the stats of the weapon. This is due to the possibility of shooting more than one projectile.