The U.S. Census Bureau does more than count people. It also administers series like County Business Patterns that help us understand our economy. County Business Patterns provides "annual detailed geographic, industry, and other information for U.S. business establishments." For more information, see About the Data.
What you'll find in County Business Patterns? You will find the number of paid employees and the number of establishments by employment size (including companies with more than 500 or 1000 employees), all by NAICS at multiple levels.
What you won't find in County Business Patterns? Businesses without Paid Employees; public administration; most government employees; crop and animal production; rail transportation; National Postal Service; pension, health, welfare, and vacation funds; trusts, estates, and agency accounts; and private households.