[Adm.Lee]

Per Wikipedia:
"In WWII, males between 18 and 64 were required to register, but the selections were limited by executive order on December 5, 1942 from from 21–45 to 18–38. Assuming the citation (George Q. Flynn, The Draft, 1940–1973) is correct, 50 million men from 18 to 45 were registered. The Selective Service statistics page gives the total inductions during WWII as 10,110,104, which would indicate that the percentage would be somewhere around 20% were drafted based on registration.

By the Vietnam war, the draft age had changed to 18-26 by the Selective Service Act. The draft pool was around 27 million (apparently lifted verbatim from a paragraph from this paper), and per the Selective Service page linked above, there were 1,857,304 inductions during Vietnam. Note that Morris gives the number as 2,215,000 - this may reflect a difference in start and end dates used but they are fairly close. This would indicate that roughly 7-9% of the draft pool was conscripted."

So, 2 million per year in a serious emergency, to bring the combined US armed forces to around 13 million at war's end. In a not-so-serious emergency, an average of 200,000 to maintain a force of over a million.

If I had to guess, the Twilight-era US forces would want to draft at least a million per year, but probably don't achieve that. Say they start in late 1996, once shooting starts, ramp up in 1997 to fill up some of the units that need to be created or filled from cadre, such as the dozen training divisions that have to become light infantry/security formations after the TDM instead of maintaining the replacement stream to the battlefronts. Assume new formations try to resume that role in 1998, but disorganization keeps things breaking down.