Under Config/Set native Units, set this to INCH if you primarily use Imperial units.
Then all code that includes G20 at the start will show in INCHES and G21 will show in MM.
Or, MDI G20, G21 to toggle between the two.
You should really go through the Mach3 Install manual section 5.4 especially as ther are other settings associated with this. Motor tuning for one.
The manual is under the Support link at the top of this page.