I’ve had them hatch under a broody and in the incubator starting at day 19. I’ve had broody and incubator hatches starting on day 21. Some hatches are over within 24 hours of the first one hatching, some hatches drag on more than two full days and nights. Each hatch is different.
There are a lot of different reasons eggs can be early or late; humidity, heredity, how and how long they are stored before starting, average incubating temperatures, even differences in individual eggs. Treat that 21 days as a target, something that is nice to hit but not something that is right on every time. This unknown can cause worries even if you’ve done it a few times.