I think I put 18 or 19 under a light Sussex last year and that was fine, I think she could have managed a few more, maybe 24. I would try more next time. Average size hen and chicks, full sized, not bantams.
But she was inside a draft free hutch, with a run with wood panelled half sides, which was also covered half wood, half glass, so was always lovely and warm, dry and only had slight breeze in even horrible weather... This may have helped cut down her work keeping them warm?