The short answer
FMCSA’s Company Census lists 2,217,311 active USDOT-registered carriers with a US physical address. That is broader than “trucking companies” in everyday speech: it includes private fleets that haul their own goods (36.4% of the total), bus and passenger carriers, and carriers registered for intrastate operation only (68.9%). Counting only carriers with an active operating authority, the for-hire companies that haul other people’s freight across state lines, gives 661,425 (29.8%).
Most of them are small. 52.9% report a single power unit, and fleets of more than 100 power units make up 0.4%; the largest trucking companies page ranks them. Each figure counts one active USDOT number; an active registration does not prove a company is operating today. The glossary explains the terms.