You can check how many employees you currently have on the first page of your journal. Both workers in your businesses and sailors on board your convoys count as employees. To increase your number of employees you need to recruit settlers from other towns (preferably viceroy and governor towns, as these will have many more settlers willing to work for you) You do this by selecting the town’s dock and transferring the settlers to your convoy. Once you begin to establish businesses, you will need to unload sailors to convert them to workers, which will free up space on your convoys for additional sailors. Continue this process as you build up your businesses and convoys until you hit a total of 15,000 workers/sailors.
This will most likely come naturally as you progress through the game but it is a good idea to equip all convoys and businesses with maximum sailors/workers.