You need workers to help you in your factories (Dairy, Pottery, etc.). Without them, you will not be able to craft any items. The more factories you have, the more workers you need.
In order to know the exact number of workers you have, tap on any of the houses for workers. There you'll see the total number of them and the number of workers that are free and available at the moment.
 
To check if you have enough workers available, simply tap on the factory you want to use. Inside the factory, you’ll see two numbers:
  • The second number shows how many workers are needed to start production.
  • The first number indicates how many workers you currently have.
If the first number matches the second, you have enough workers and can start production. If the first number is lower than the second, you don’t have enough workers to begin and you either need to wait for the production in other factories to finish or increase the total number of workers.