Agent Ranks (Apocalypse)

Promotion Eligibility: Rank Prerequisite Rookie None Squaddie Must earn over 8 combat rating points Squad Leader (Rookies + Squaddies) / (Squad Leaders + 1) is more than 4 Sergeant Squad leaders / (Sergeant + 1) is more than 2 Captain Sergeants / (Captains + 1) is more than 2 Colonel Captains / (Colonels + 1) is more than 2 Commander No commander, and more than one Colonel Up to five units may be promoted after any given mission. The units with the highest combat rating are selected for promotion.

In affect, this means that you need to hire:

5 soldiers to get 1 squad leader +4 Squaddies/Rookies 15 soldiers to get 1 sergeant + 2 squad leaders +12 Squaddies/Rookies 37 soldiers to get 1 captain + 2 sergeants + 6 squad leaders + 28 Squaddies/Rookies 83 soldiers to get 1 colonel + 2 captains + 6 sergeants + 14 squad leaders + 60 Squaddies/Rookies 130 soldiers to get 1 commander + 1 colonel + 4 captain + 10 sergeants + 22 squad leaders + 92 Squaddies/Rookies

Apparently it is impossible to hire 130 in Apocalypse and thus you cannot get a commander.