decals are like banners, you can place them on top of any other texture without having to worry about z-fighting. (gfx bug that happens when the engine does not know which texture it should draw.)
i dont know yet if there is a limit for it, or if you should not use too many.

create a brush, and choose the right 2d-view for creating a patch mesh via shift+p, and create the mesh. choose your texture, maybe set it correctly via the surface inspector (shift+s), and place the mesh near the place where you want the decal.
create an info_null (?) entity behind the mesh (has to be sealed off of the void, either place it somewhere within the map, or create a box around it, else you will have a leaked entity. leftclick the mesh and the entity, and connect them (ctrl+k).

finished.