By my count there were 11 divisions in Poland and East Prussia in June 1940. 18 Army consisting of the 208, 216, 225, 227, 251 and 254 Infantry divisions were the "Polish reserves" attached directly to OKW. And the XXXIV and XXXV Corps consisting of the 209, 365, 372, 379, 393 Infantry divisions were the OKH reserves located in East Prussia.

In addition there were another 27 OKH reserve divisions across Germany (including Austria and Czech territories. There were also seven divisions in Norway.

Hitler didn't leave his Eastern border completely unprotected. But he didn't leave much. For comparison he left 20 divisions facing France during the attack into Poland. But the Soviets weren't mobilizing so there wasn't an immediate apparent threat.