How many brand names of semi trucks are there?

Answer 1

In the US/Canadian market, there are eight active manufacturers. They are:
Freightliner (subsidary of Daimler A.G.)
Western Star (subsidiary of Daimler A.G.)
Kenworth (subsidiary of PACCAR, Inc.)
Peterbilt (subsidiary of PACCAR, Inc.)
Mack (subsidiary of AB Volvo)
Volvo (subsidiary of AB Volvo)
International (subsidiary of Navistar, Inc.)
Caterpillar

And several marques which have been discontinued, but which might still be seen on the road.
Sterling was a manufacturer bought out by Volvo in the 1950s and discontinued. The name was revived in 1997 by Daimler A.G., and was discontinued again in 2009.

Ford left the heavy truck market in 1997, selling its production line to Daimler A.G. (who put the into production under the Sterling name).

General Motors left the market in 1988, discontinuing the Chevrolet line in 1980, and the GMC line in 1988, and the line was sold to AB Volvo. However, they still produced the Top Kick - which could be configured as a single axle truck tractor - until 2009.

Marmon closed its door in 1997.

White was bought out by Volvo, and the name was discontinued sometime in the 1990s.

Scania (a Swedish manufacturer) attempted to break into the US market in the early to mid 1980s. However, they failed to establish a nationwide network (with the only dealership being in New Jersey), used their proprietary drivetrain and axles (rather than the typical Dana axles and Eaton Roadranger transmission), and the trucks were considerably heavier than US trucks. They gave up on that venture in the US and Canada by 1990, although they sell trucks in every other part of the world.