I am no plumber, but I have been in the trade long enough to see this type of need arise.
Most hot water radiator systems are in a "closed loop", that is the pipe that feeds the radiators goes from the boiler, to a pump, then to the various radiators on that loop, then return back to the boiler. There can be more than one loop. These loops are called zones. Each zone may have one to 4 or more depending on the heat loss that each radiator emits doing it's job before the water returns to the boiler to be heated again. There is a formula/calculation that plumbers use to determine how many radiators a zone can handle.
To bleed the system there is a faucet to drain the system (zone) completely, the other zones need not be affected. This part I am rusty on, Knowing what type of system you have and how old it is would help... for there are other valves that need to be closed (main water supply feeding the boiler, NOT the whole house) if the system is tied into your domestic hot water, as some will have that feature.
This may generate enough info to you to get you started, and have another member chime in...
And... to answer your question, it will be gallons, the amount that is in the line to the radiator and the other ones on the same zone.
The volume/quantity really should not matter since there is typically a garden hose attached to the drain valve or faucet, and that is run to a floor drain, sink or toilet, lower in height than the drain valve. You will hear the water passing, and after a time, say a half hour or less the water should have drained out, unless there is a supply still feeding it that needs to be cut off.