There is a certain tactical rotation in occupation wars. Troops cannot fight continuously. Soldiers should rest and not be in a constant conflict environment. When we look at the captured soldiers, it is seen that most of the captured Russians were not professional soldiers. Psychological management is more critical for these personnel.
We don't know the exact number, but as a result of the troop movements tracked from open sources, it was estimated that there was more than 120.000/ less than 200000 ( full size army) troop placement just prior to the operation (along the entire ukrainian border line, Crimea + Belarus).
What is misunderstood here is that it is thought as if over 100000 Russian soldiers were attacking or going to attack at the same time...