Recruits are a vital resource in Empires & Puzzles. The number you have is shown near the top of the screen while viewing either the map or stronghold.
The training camp is used to train recruits into heroes.
Housing & Storing Recruits
The maximum number of recruits you can have at any one time is determined by your houses. However, once a recruit has been sent to the training camp, it no longer counts toward your total, so many players store extra recruits in TC4 or TC11 (Training Camp levels 4 or 11). This allows them to avoid upgrading their houses, so they can use the resources elsewhere.
Eventually, you will want to be able to house at least 100 recruits in your houses at one time in order to run TC20 for a chance for some Legendary 5* Heroes.