Realistically, the solomani are about the only OTU power likely to have more than a few... since they take a -1TL vs gravitics (source from CT era...) in exchange for +1 TL for medical tech.
So... they have a 2 TL range for mechs...
Seriously, though, the real world applications are NOT typically going to be combat. The applications are, however, unusual: Construction, Mining, forestry, dockyards. (Remember: The loader Ripley uses in Alien is based upon a real world prototype...) The scandinavians are using 6-legged vehicles for logging. Loaders have an advantage of being intuitive to operate if bipedal with 2 arms. Same for construction work and mining work. Additionally, multi-legged vehicles are ideal for mining due to the excellent steep and rough terrain handling, plus the ability to (if properly designed) brace ceilings.
Walkers ARE NOT FAST... at least not by the design sequences in Traveller and GURPS. They top out at about 60 MPH... Treads can go faster. Treads, however, have trouble over 40-60 degrees of slope. Wheels between 30-60 degrees of slope (basically, more wheeles, and right tires, and you can make it happen...) ... with the right equippage, however, walkers can traverse up to 75 degree slopes (NASA proved that some 50 miles from where I sit... a volcanic caldera in either Mt. Redoubt, Illiamna or Spurr...). With the right equipment, a walker could be made to climb 90 degree slopes... but reasonably, about 75 to 80 degrees is about all one could hope for...
Tracks have a problem in forestry of tearing up the soil. Wheels have the same problem. Walkers don't, and actually are being designed with LESS ground pressure than wheels, and comparable with tracks...
The other advantage over tracks is less critical part contact with debris...
But none of the real world walkers are being designed for "GIANT robot" modes... the loaders are about 10-12 feet tall, and have about 1 to 2 square meter footprints, and move a measly 10 MPH... with lifts of up to about 2 tons...
The scandinavian forestry walkers (still experimental, but functional) have foot pads about 40cm in diameter, and have 3-5 pads in contact at all times, dependant upon speed and load.
Combat walkers would probably be either loader based, with extensible legs for speed... making them comparable with some of the slower tread tanks... again, intuitiveness, not weaponry, would be the key gain, and, with proper design, lower ground pressures and less obvious tracks possible. By tying movement to leg motions and weapons to arm motions, one could make the single-man light tank... essentially the idea behind Heavy Gear's Gears... and have far more terrain open to movement.