We don't really have cities the size of New York. At all. Clostest cluster of cities is Cologne, Düsseldorf, Bonn, Dortmund - within an hours drive. But there are two German indoor slopes in that general area as well.
Four major groups of visitors: absolute beginners getting a feel for the snow or taking a few lessons; one time-visitors (birthday parties etc.), professional race teams (from all over Europe, especially in juli-september, when many glacier areas in the Alps are closed as well); and experienced skiers and snowboarders loooking to stay in ski shape (me).
As far as I know, there is no limitation of how many people they let in. It can be insanely crowded too, never experenced that myself though (plan well, check in advance for closure of pistes or big events).
Here you can see for yourself, they have a few live webcams: http://www.snowworld.com/en/Ski-slopes/Ski-Slopes/Webcams-SnowWorld
Zoetermeer is the shorter, steeper venue (run 3, now without snow for the expansion), Landgraaf is the longer slope, piste 1 being the FIS-rated one.