Interstate carriers operating commercial vehicles with a GVWR or GCWR of 10,001 pounds or more need one, and some states require it for intrastate carriers as well. The number goes on the sides of the power unit.
A USDOT number is not operating authority. A for-hire carrier that hauls regulated freight across state lines also needs operating authority, shown as an MC number.
Regulation: 49 CFR 390.19.