Who works
Most buildings get more workers with every level. A Mushroom Farm needs no workers. The number at the top of your town shows your workers.
Room for workers
The Engine Hall sets how many workers your town can have: 9 at level 1, 16 at level 2, 29 at level 3 … 1045 at level 20. When there is no room, you cannot build or upgrade a building with workers. Upgrade the Engine Hall first. You can always upgrade the Engine Hall itself.
Food
Each worker eats 1 Food per hour. You can only start a building with workers when your Food covers every worker. Build and upgrade Mushroom Farms for more Food.