How many hazard classes make up the U.S. Department of Transportation's hazardous materials classification system?

49 CFR 172.101 Hazardous Materials Table, Subpart B; AAMVA CDL Manual (nhcdm.pdf) Figure 2.24, p. 2-42

Even drivers without a hazmat endorsement need to recognize the hazard class system, because it determines what placards they'll see on other vehicles and what basic precautions apply if they're hauling a small, non-placarded hazmat shipment. Misidentifying a hazard class in an emergency can mean using the wrong response and making a situation more dangerous.