how much voltage can one radish produce?

One by itself? Probably not much. Not sure we can answer this - in an empirical question ripe for an experiement  you could have a crack at yourself. Try switching the spuds for radishes in this expt and see what happens. You'd need to find a voltmeter too though I guess.

http://www.homemade-circuits.com/2012/0 … -from.html