While I was unfortunately unable to find any specific information as to how many of the enlisted members of the early precursors of the JSDF (Jieitei) served in the Imperial Japanese armed forces (to my knowlege such records were not reliably kept), it's likely that a portion of the 75,000 members of the National Police Reserve had served with the IJA at some point, though how many exactly is hard to say. However, the officer's corps is another story. It was originally comprised mainly of reserve officers, as professional NCOs and officers were excluded from service, but due to the expanding war in Korea and a need for experienced officers in the Japanese defense forces, this policy was eventually revoked. In total, more than five thousand regular officers who had graduated from the former imperial military schools eventually entered the military in the Police Reserve and the early Self-Defense Force years.

also referenced:

• Sasaki, Tomoyuki. (2009). An army for the people : the self- defense forces and society in postwar Japan. UC San Diego: b6639848. Retrieved from: http://escholarship.org/uc/item/58n4d7wb

• Maeda, Tetsuo. (1995). The hidden army: The untold story of Japan's military forces. Chicago: Edition q.