Not true. A circuit's rating is determined by the rating of the breakers, not by the actual connected load. But the limit of what you can put on a panel is determined by the connected load, with any allowable demand factors applied. You calculate the load without ever knowing, or caring about, the rating of the breakers. Breaker selection is a design decision that comes later in the process. You can put 42 circuits, each rated 20 amps, on a 100 amp panel, provided that the total calculated load does not exceed 100 amps.