Interesting question. When I was a kid we had a hen hide a nest and bring 18 chicks off. I have no idea how many eggs she started with, I never did find that nest. With my flock I had one hen raise 15 chicks, another raised 16. These were both a mix of chicks the broody hatched and incubator chicks. I'm not afraid to give a hen a lot of chicks but there are things I consider.
One is the time of the year. How warm is it. In fairly warm weather the hen doesn't necessarily need to cover all the chicks at night, often some chicks in a fairly small brood will sleep next to the hen instead of under her. In colder weather she needs to be able to cover them. They grow pretty fast. She may be able to cover all of them one week but not the next. Even with smaller broods the chicks can get too big to all fit under her at night but by then they can handle the colder weather. People on this forum often underestimate just how well a chick can handle cooler temperatures but there are limits.
Another consideration along the same lines of her covering them. A broody hen can cover a lot more bantams or smaller breed chicks than she can full sized chicks.
Can a hen keep track of that many chicks? I don't think there is a clear answer to that. Some do better than others. Some chicks stick pretty close to Mama, some are more adventurous and may need to be kept track of. I think this depends on the personality of the chicks as much as the ability of the broody. It's not so much a numbers thing as just luck.
I don't know the right answer for you. She may lose some of those 17 chicks. She may have no problem with all 21. Unfortunately there is no way to predict it with any accuracy. I probably would not try it, it may be stretching it a bit much. But I've been wrong before.