there is a library research ‘free companies’ but thats the very first research you’ll see so most likely you’ve already researched it.
other than that, upgrading the camp is the only way, its just that some upgrades dont provide extra space, they just provide better quality. camp level 11 will only give you 6 merc space. 7 for chinese. there are also other upgrades like airfield and war academy that dont provide any extra space with some upgrades
