You can't buy the military discounted
MNSSHP tickets ahead of time, such as by phone, online or at your base. You have to buy them on WDW property or at Shades of Green resort at WDW. I have purchased the military discount MNSSHP tickets at DTD (Disney Guest Relations) approximately a week before the party we were going to. I think that's the "technical" limit of buying in advance -- on WDW property. If you're worried about the event being sold out on your night, the military member may buy tickets for sold out nights. Both links describe where and when to go to get those tickets. The reservist ID is all the National Guard/Reserve member has, right? Guess that will have to be the ID to use. The following 2 links will give you the info you're looking for --
The Mouse for Less .com says a soldier may buy, for him/herself and up to five guests, tickets for one party. It's not said, but I know from reading elsewhere, that if you plan to attend more than one party during the discount month, the discount offer can be applied to multiple party nights. Mouse for Less also mentions the party discount is offered to active National Guard and Reserve members with military id.
Active Duty, Retired, and Permanently Disabled US Military, including members of the U.S. Coast Guard and
active members of the National Guard or Reservists,
can purchase discounted tickets for themselves and up to 5 family members and/or friends for select dates. The tickets may be purchased in person at any theme park ticket window, guest relations location or Shades of Green with proper military ID. For sold out parties, Military discounted tickets for MNSSHP will be available
only at the Magic Kingdom Front Gate Guest Relations window on the day of the event. Discounted tickets are NOT available online, by phone or at MWR/ITT locations.
http://www.themouseforless.com/walt.../events/mickeys-not-so-scary-halloween-party/
Military Disney Tips .com has great info about the MNSSHP party. They didn't mention number of tickets that a soldier can purchase, per party. They do mention eligibility for active National Guard and Reserve members with military ID.
