If you add all their active, reserves, and paramilitary troops, they can get to 3.5 million. That would leave them defenseless on all other possible fronts, and many of those reserves and paramilitary troops are not fit for combat. Many of the active troops are not in parts of the military that can be used in Ukraine.
To get to 6 million, they would need to draft every male between the ages of 18 to 28, which they could not do. They would be lucky to get half the males, so would have to draft half the males between 18 and 38.
It is illegal to even say that Russia is at war. You cannot draft a huge chunk of your population for a "not war". Putin would collapse if he tried to get 6 million troops out of Russia.
Maybe there is another option that we are not seeing, but Putin is. Having said that, it does not currently look good for Putin.