How do I increase the number of workers in an industrial area?

"Given that I am still at level 1 and only have one of each type of building, does it mean I have to build multiple plantations and sawmills?"

Correct. Also ensure that:

  1. Your city has industrial demand, so that workers will actually fill up open industry jobs.

  2. Road connections to your industry buildings are not blocked by traffic, destroyed, flooded etc.

  3. Industry buildings are not operating at reduced capacity. All utilities (electric, water, sewage) should be provided.