Here's how to work it out.
In parallel (which pretty much all amp outputs and daisy chain sockets on cabs are), you work it out like so. For two 8ohm cabs:
1 / (1/8 + 1/8) =
1 / (0.125 + 0.125) =
1 / 0.25 =
4
If wiring in series, then simply add them, i.e., 8 + 8 = 16.