For a heavily-militarised modern-day society - and with the number of wars in ED, most of them probably are - somewhere between 0.1% and 1% of the population as active duty soldiers is a normal range, plus rather more variable numbers of reservists. So for the billion-resident ELW somewhere around a few million active duty personnel, and a few tens of thousands of spaceships, seems reasonable (ownership split between the various factions, of course).
For the 50,000 outpost, that same percentage would give 50-500 soldiers split between the various factions, which probably in practice means that there's no standing force as such: internal 'wars' will be "two groups of drunk rowdies having a punch-up" up to "two security teams have a brief firefight", if the system as a whole needs defending it'll be "whoever has an armed ship tries to make the best of it while everyone else tries to barricade and use the Pioneer Supplies stock for when the boarding parties get through"