How many eight ga wire will fit into three quarter inch pvc?

The legal limit is somewhat lower than the physical limit. The number allowed depends on the insulation type and the current carried by the wire. One table I saw put the limit at 5 wires of size AWG 8 in 3/4 inch conduit. Consult your applicable electrical code.

Ref. infohost.nmt.edu/~armiller/homerepair/conduitsize.htm

The diameter of AWG 8 (American Wire Gauge) is .128496" (bare wire) if this is just a mathematical problem.