The rules in plain English
Construction: 1 employee
A construction business needs workers' comp as soon as it has one employee. Corporate officers and LLC members who own at least 10% count as employees. Up to 3 of them can file a construction exemption ($50, good for 2 years) if each owns at least 10%. A fourth officer can't be exempt. Construction sole proprietors and partners can't be exempt either, so they count.
Everything else: 4 employees
Other businesses need coverage at 4 or more employees, full-time or part-time. Officers and LLC members count unless they file an exemption, and any number of non-construction officers can. Sole proprietors and partners don't count unless they choose to be covered.
Farms: 6 regular or 12 seasonal
A farm needs coverage with 6 regular employees, or 12 seasonal workers who work more than 30 days in a season.
How to look up a coverage or exemption
The state's Proof of Coverage page has two public searches. Exemption Search shows corporate officers and LLC members who hold a Certificate of Election to be Exempt, and Coverage Search shows a business's carrier, policy number, and coverage dates. Search by employer name (at least 3 letters) or FEIN. A new exemption appears the day after it is issued. A business with neither, and enough employees to need coverage, is out of compliance.
What happens without it
The state can shut the job down with a stop-work order. The penalty is twice the premium the business should have paid for the uninsured period in the last 12 months, or $1,000, whichever is more, plus $1,000 for every day it works under the stop-work order. Source: Fla. Stat. 440.107.