In Central America, most of the small number who are here are in Costa Rica or Honduras, not Belize. And Mexico has many hundred of times as many expat Americans as all of Central America.
I just wrote a book on living/retiring/investing in Belize, and the best estimate I and the people I interviewed could come up with was around 2,000 expat Americans, Canadians and Europeans in Belize, and that includes snowbirds who are there on tourist cards. One knowledgeable expat put the number of foreign residents in Belize as no more than 600, but with all the foreign hotel operators I'd put the number higher than that. The number of permanent status foreign residents is smaller. For example, only 100 foreigners so far have been approved for residency under the year-old Qualified Retired Persons Incentive Act.
Two reasons more military don't retire in Central America are medical care and lack of PXs. A lot of retired military congregate near U.S. military bases -- and there aren't any in Belize.
Expats in Belize are mainly in San Pedro, Cayo, Placencia, Belize City and Corozal Town, with a few missionaries and intrepid souls elsewhere, mostly in PG and Hopkins.
Belize First Magazine http://www.turq.com/belizefirst/