As far as board houses are concerned "double sided" means there are components on both sides. Only the simplest and cheapest of boards are 2-layer, or the top and bottom surface are the only metal layers. Typically boards have 2 plane layers (power and gnd) usually towards the center or edges to balance the copper content, and then around 2-10+ routing layers depending on the density of the nets. Of course there are a number of different stack-up possibilities.

I'm impressed the empeg card is only 6 layers. To me, it looks like it'd be at least an 8, must have been a skilled place and route guy that doesn't like to waste layers :)

#080000512 green 12GB