there is no rule of thumb. it depends on how many slots there are, and junk.
consider an old, 1 agp, 5 pci, 2 isa (one shared).... the 2nd through 4th are bus mastering.
however, with newer pci chipsets, all can be bus mastering...
um... it's safer to consider the 2nd and 3rd to be truly bus mastering. 4th and 5th share, and 1st and agp share.