He didn't say "all." He said "one or several."
Jim, if they are incandescent, AND there are 20, AND they are in series, then each one should run on 6 volts.
HOWEVER, the best way to do this is to use a voltmeter and see what the voltage drop is across a typical lamp. They might be in a series/parallel configuration, or some other weirdness. Use science, first, before letting out the smoke.