Some "rules of thumb" about selenium rectifiers:
The forward voltage drop should be about 1 volt per plate.
The reverse blocking voltage is about 25 to 30 V rms per plate.
Rectifiers for 117 V operation usually had 5 or 6 plates.
Most all designs used a current-limiting resistor in series with the selenium rectifier, usually 20 ohms or less.
A one inch square plate will handle about 100 mA. avg. DC current.
You may find rectifiers with many plates, but these usually have a tap for use as a center-tapped full-wave, as in a battery charger, or a voltage doubler, common in TV sets of the late 50s.http://www.w3hwj.com/index_files/RBSelenium2.pdfhttp://www.pmillett.com/file_downloads/selenium.pdf