Dailies are added periodically throughout the day so the durations you see at any moment in time are varied, but I think they all start at 24 hours when new.
I don’t think there’s a fixed number of dailies per zone - it seemed to me like some days one zone would have more and the next a different zone would have more. There might be a fixed number of total world quests across all the Shadowlands zones combined, I never bothered to keep track.