To a large extent the system's capacity is shared among all active aircraft. Yes, it matters somewhat how many people on your plane are using the wifi. I believe what matters more is the total usage fleetwide at that moment.
correct--capacity of the access point radios in the plane would first show issues on streaming tv/movies. approx. 100 people could pull a 4mbps video stream given 4 radios (and I doubt WN's stream is much over 2mbps).
I have no idea what the max. design capcity for one external Row44 satellite antenna. ROUGH guess is 20mbps (simply because that is what I could pull years ago with only one WN Wifi aircraft). Back in those days you could watch your own tv through a slingbox, watch youtube in HD, etc.
I'd be surprised if they have enough overall capacity to give 512k-1mbps per wifi aircraft.