the compaction is minimal when considering quantity for a normal residential patio. The differential begins to factor in when you are talking very large areas. You want a minimum of 4" depth (6" is preferable). I personally use stone dust, but you can use a specialized sand.
For yards of material: Calculate the patio area in square feet. Then multiply by 0.5 (6" depth). Then divide by 26.99 (1 cubic yard = 26.99 cubic feet).
For example: a 20'x20' patio would need 7.4 yards of base material
[ (20 x 20) x (0.5) ] / (26.99) = 7.4
If it is sold by the ton, tell the supplied how many yards you need and they should now what tonnage of their product it would take to equal that.