I believe the strength of the remaining 4 studs would be sufficient for "most" driving conditions. Just keep your speed reasonable and try not to do any violent cornering maneuvers until you can get the missing stud replaced.
The rim itself does not really ride on the studs. The axle shaft has a raised collar on it. The center of the rim fits this collar. The nuts on the stud keep the rim square to the axle shaft. So the load is really on the rim center, not the studs.