Each layer can have a layer mask (raster) and a vector mask. With the transparency of the layer, it makes 3 masks.
You can use another layer below that layer and create a clipping group (the opacity/transparency of the lowest layer will be applied to the other one) by ALt/Option clicking on the line that separates those two layers. The clipping layer can have both a layer mask and a vector mask. That makes 6 masks.
You can place those two layers in a set. A set can have both a layer mask and a vector mask. 8 masks total.
In CS (Photoshop 8) you can have 5 levels of sets. 16 masks... and I don't count the blending modes or the avanced blending options...
Pierre Courtejoie