It's not an exact science, 3:1, 5:1, highly dependent on morale, local conditions and so on. But defenders have an advantage assuming equal weaponry.

Russia has air superiority so that definitely upsets the balance, but so far they have not been able to use that very effectively. They did not even manage to protect their troop transport airplanes all that well, two of which got downed, and that really adds up in casualties.

It's a combat rule. Known from Sun Tzu, and later.