To mobilise a million men Russia needs the men to train them, the supplies to equip them and the tanks, armour and artillery for them to operate.
The Russian ground forces constitue about 400K, of whom 200K are conscripts and who weren’t supposed to go to war. The number of troops sent into Ukraine is assessed to be about 190K.
Most of those will have been the professionals. About 25-30K are dead and around another 60K+ will be wounded. The rest can’t be pulled out without the entire war and army collapsing.
Back home they can’t produce new tanks or artillery because they’re factories are not set up for mass production and they need electronics which are sanctioned.
Frankly, any mobilisation is liable to be even more of a fiasco than war as those at the top realise that the corruption, embezzlement and hollowing out of the needed infrastructure is even worse than in the supposed “reserves” of tanks left rotting in fields and trucks with tyres over 20 years old still stamped “product of the USSR”.