Here are some numbers for Lionel and MTH.
Lionel Scale O Car: 300 units
Lionel Traditional Scale Car: 500 units
Lionel "Made in USA" line: 100 units
Lionel "Lionscale" (former Weaver Tooling): 100 units
American Flyer S scale: 300 units
MTH O scale: 256 units
MTH Tinplate: Varies, but usually around 100 units. If you don't mind waiting, they sill sometimes run less if it is run when run the catalog version.
Good luck with you project!