I'm pushing 10 different Google calendars to multiple devices. Haven't tried any more than that.
Events added to any of the calendars online at google.com/calendar push to default calendar on the BB (as stated before), not allowing for color coding of calendars on the device - a nice feature I managed once, but too cumbersome and not imperative.
The only hitch is that adding an event on the device pushes to a single calendar at Google - I have set my device to not add new handheld events anywhere, as I daily add and edit events on several of the calendars. I would rather make a note in my tasks and add events when I get to a PC, so that the event shows up on the right calendar at Google.
Very interested in seeing this feature developed more fully.