> Ukrainian estimates put their potential there at 20 million soldiers
That is pratical only in the event of total societal breakdown like Nazi Germany in its final months when there was no economy left and institutions like schools, hospitals, factories and utilities had ceased to work. Everything was thrown to the front while the people in cities were malnourished, had no power nor running water, and died of minor diseases due to lack of medical supplies.
Putin is too afraid to even announce a regular mobilization and resorts to dirty tactics coercion of poor rural ethnic minorities to fill the ranks, because the backlash that would follow a general mobilization would be a major threat to Putin's grip on power. Look up the effect of soldiers' mothers movement during the Chechen wars, or how the forced conscription into the Soviet-Afghan war (620k people over a decade) contributed to the disintegration of the USSR. Educated middle-class people accustomed to European lifestyle won't go to the front to get slaughtered like IQ 80 villagers who are up to their necks in payday loans; they will resist and cause major disruption among the productive urban population that keeps the country running.
These millions of soldiers exist only on paper. The actual situation on the ground is pretty dire. Ukraine hit another all-time record yesterday, killing or disabling 2250 Russians in a single day. The entire month of October is trending to hit a record too, 50k casualties in a single month. Russian recruitment target is 34k per month.