If someone can tell me how many deployable Formations or Sub units, and of what types and capabilities, the US Army/USMC needs, then I think there is the bones of a discussion, but absolute numbers are meaningless.
Will, that is the challenge. It is rooted in the discussion about what you want your military to be capable of doing for you as an instrument of policy, and what kind of results are you willing to live with on the back end - which is generally the side that contains more unknowns. In this case it has an interesting twist, because while we know we'd like to have to other USG agencies doing some of these unknowns, the military remains the "tool" most likely to be doing them for a number of reasons. This is made somewhat more difficult because the capabilities to meet some of these types of requirements are not collective based, but resident in individuals.

I know allot of smart folks who have produced numbers - they all strike me as having some subjectivity in them, i.e. they are estimates. This is not bad, war and policy has a way of resisting business like efficiencies - it has always seemed like a good idea if you can have enough to keep something out of contact to retain your flexibility. The resulting question is can you afford it/live with it, and if not, can you sacrifice something else to get there? Its hard for folks (a population) to rationalize keeping something on hand when they cannot understand why, or how it relates to their interests in contrast to other seemingly more immediate needs. This may be more true when we say we not only need to recruit good people, but as we train, educate and develop them, we must find ways to retain them and their families.

A professional military takes more time and resources to build and sustain then a conscript one (relative to size and not accounting for costs brought on in respect to effectiveness and efficiencies); however a professional military can do things "differently" that a conscript one.

A conscript expansion offers some flexibility, but you have to be able to live with what the implications and consequences are on a number of levels to rely on conscription to fill out/augment your base formations. Again, what do we want our military to do, and can we live with the notion that regardless of what we thought we wanted it to be able to do 5 years ago, new policy requirements will inevitably create situations that were unanticipated and don't fall neatly into that established lane. How much is the flexibility to better achieve your objectives worth? How much risk are you willing to assume and where?

Best, Rob