Actually they CAN build up to 20.
Heck they can build up to 10000000000000(add incredible amount of zeros here).
If you didn't notice yet, conscript don't count towards pop cap UNTILL you upgrade them to NKVD conscripts at which point the comissar costs 1 point of population.
Right now IMO there a serious lack of limit on many things.
Now don't get me wrong, I love the soviets and I'm currently playing them and won/lost a few games against players or CPUs and this lack of limits gives me or any other soviet players a major advantage.
There also that other big ass tank from the last doctrine on the right. Can't remember the doctrine's or tank's name but it costs 1000 manpower to call and only counts for 12 for pop cap. There is no limit on how many of that thing you can call. Or at least none that I could see when I had 3 of them on the field. At that point I was dominating the battlefield, not even a king tiger stood a chance. Same goes for the IS-2, that thing is basically a pershing tank with increased armor/health. Same damage but from what i saw a rank1 pershing was easier to kill than an IS-2 and there is no limit on that thing either.
Of course people need to give the EF's dev team time to figure out what to do and any immediate and major change could totally gimp the soviets. Hopefully they will take their time to figure out everything and make the appropriate changes.