The Red Army had 100,000 troops in Manchuria fighting the Japanese. They couldn’t have gone against Poland on September 1 if they had wanted to.
In 1939 the Red Army strength was about 1.8 million men under arms. Late in 1939 the Soviets employed 600,000 men in the war with the Finns. They had the manpower to attack Poland if they wanted to, but the elected to let the Germans take care of the Polish army for them.