There is no limits on the number of CBs we can put on any one feeder. Twenty, fourty, eighty, it doesn't matter. What does matter is the computed load. If your computed load, using the rules found in the NEC, is less than 100 amps, then that 100 amp service or feeder is just fine with the 42 circuit, 200 ampere panel (with or without 200 amp main), if fed with a 100 amp feeder, and protected by a 100 amp main CB.
I have installed up to four, 42 circuit, 200 amp 3 phase 480 volt panels on a single 200 amp feeder, using double lugs on the first three panels. The panels were used for switching lighting, (SWD breakers) and were spaced throughout the building (a high school) at convenient locations in groups of two.
I was an apprentice at the time, and I questioned the combined loads of all those CBs. The journetman took the time to discuss the concept of loading, diversity of loads, and computed loads with me. It was a valuable lesson.
He also indicated that it did not matter where any circuit breaker was on the bus, but he preferred to have the higher ampacity ones closer to the main. It just felt better.
