War time resources dictate the need for an active reserve force though not all nations are capable of carrying active reserve formations. As with active frontline personnel, these numbers tend to favor those nations composed of a large base population, large defense budget and (often times) currently active operations.
Active Reserve Military Personnel data through 2016.
Sources: CIA.gov, CIA World Factbook, wikipedia.com, public domain print and media sources and user contributions. Some values may be estimated when official sources are lacking.