It is 2 filaments - however, the two filaments are of comparable brightness - not like a brake light where they are of greatly different brightness and construction. Low and high filaments are nominally 55 and 60 W respectively.
I'm only familiar with the H4 type of bulb - in this, the low beam filament is surrounded by a reflector which directs the light down towards the road. The high beam filament does not have an integral reflector, and so the light output is undirected. When the high-beams are switched on, the low filament must be switched off (otherwise the bulb will overheat). Take a look at the photo over
here