IBM offers 256 channels per system and Amdahl and Hitachi support up to 512. Secondly, instead of one pathway into memory as in a PC, there are multiple memory banks providing multiple ports into memory. For example, Hitachi's Skylines have 16 ports into main memory and 32 ports into cache memory, and that cache memory is 10 times faster than regular, main memory. Thirdly, while a 200MHz Pentium has an internal bus of 66MHz, a 200MHz mainframe may have a data bus that also runs at 200MHz, three times as fast. Multiply one times the other, and the combination makes a processing machine unlike anything else.
Mainframes are very scalable.
To increase capability beyond a single system, IBM uses its Parallel Sysplex technology to cluster up to 32 systems, making them all appear as a single system image. Amdahl recommends no more than four or five. It's a tad harder than crashing Windows, which just about anybody can do in 10 seconds. NT may be more stable than Win 95, but NT has had barely five years in the trenches. Half the hardware in a mainframe is error detection and correction circuitry.