Do they have a system that pumps the air out or does it just let it out?
I haven't found a reference giving the number, but we can make a back of the envelope approximation.
Page 49 of The International Space Station - Operating an Outpost in the New Frontier reminds us that the crewlock on the ISS is the same volume as the Shuttle Orbiter airlock.
From work experience as a Shuttle Systems instructor, I know
So the air volume in airlock with 2 suited crewmembers present ~ 165 cubic feet
(If you are wondering about the 228 cubic feet in the table vs the 185 cubic feet in the diagram, the difference is the 43 cubic feet of the tunnel extension, which the ISS crewlock doesn't have)
Assuming normal conditions of 1 atm and 72 deg F the ideal gas law gives us ~ 12 lbm or 5.4 kg of air in the airlock at the start of the depress.
This answer How Is The Quest Joint Airlock Operated? tells us that the ISS airlock depress pump scavenges ~ 10 psi of the air. So ~ (5/15) * 5.4 kg is lost or in round numbers, 2 kg.
(Source of images: SMS Systems Console Handbook)