You won't find any published information, at least not any that is reliable.
The Army determines its needed number of physicians for any specialty based on deployment needs. That has very little, if any, correlation with the number needed to care for all of the beneficiaries. It makes up for the difference by constantly tweeking the number of active-duty docs, number of civilian DoD docs, workload, and number of patients pushed to the "network", meaning the local civilian healthcare system.
In general, I've found the Army is notorious for having wild shifts in its manpower. They suddenly realize that they're 10% "overstrength", so they don't renew a bunch of civilian contracts, force retirement of older physicians, and cut the number of residency spots. Then they wonder how they got to be 20% "understrength" 6 six years later and they're spending millions of dollars to push patients to the network.
The trickle down is that there is no set table of X number of active duty physicians will be assigned to Y department. Your best bet is to find a knowledgable physician in your department after starting residency, and getting specialty-specific information. Better yet, find out who the consultant is for your specialty.
Your last year in residency, assuming you're not doing a fellowship, a list of available assignments for your specialty will become available. In general, you will submit your wish list, and then you get your assignment a few weeks later. Most consultants will take your wishes into consideration, but the overwhelming majority of immediate post-residency physicians do not get their top choice or an assignment that is considered desirable.
As far as relocating, yes, it is possible, again largely depending on your specialty's consultant. As a rule, Army tours are for 3 years, but they get shortened and extended for medical corps officers all the time. Don't understand the hassle of moving. Meaning, if someone owes 3-4 years, it's generally not considered worthwhile to move after 2 years, although plenty of exceptions exist. Be very careful that you do not mistakenly extend your obligation by accepting a new assignment.
I would encourage your fiance to get used to disappointment.