According to the AAMVA Commercial Driver's License Manual, how many hazard classes exist under the federal hazardous materials classification system?

AAMVA CDL Manual, Section 9.3.1 and Figure 9.1 (nhcdm.pdf p. 9-2/9-3); 49 CFR 172.101 Hazardous Materials Table

Every other hazmat rule — placarding, labeling, segregation, shipping paper format — is organized around which of the nine classes a material falls into. Knowing the count and structure is the foundation the rest of the hazmat knowledge test builds on.