How much land do you need to hunt with gun in mo?

None. You do need to have the permission of the owner of the land you're hunting on and to have a valid hunting permit (and possibly additional permits/tags for hunting certain types of game), but you don't actually need to own any land yourself.

You may be thinking of the fact that "resident landowners" are exempt from certain regulations when hunting on their own land. In that case, the applicable section of the Missouri Code of State Regulations defines a "resident landowner" as a Missouri resident who owns at least 5 acres of land in one contiguous tract, or any member of such a person's immediate household whose legal residence or domicile is the same (and has been for the past thirty days).