Grenadier was the lowest rank in the Heer, the army of the Third Reich. It was also known as Schütze. Although I have been unable to determine how many soldiers in the Heer had this rank, it can be expected that it was at least half of them, meaning roughly 1 800 000 in 1939 and as many as 3 250 000 at a time in 1943. As a comparison, 24 620 748 German men were aged 15-65 in 1939.
The Luftwaffe equivalent to Grenadier was Flieger, the Kriegsmarine equivalent was Matrose and the Waffen-SS equivalent was SS-Schütze. The equivalent US Army rank is Private.
For a more complete overview of which ranks existed, see Wehrmacht ranks.
The page that helped me with this writeup is no longer on the internet, but it used to live at www.wehrmacht-association.com/Open_Pages/ranks.htm