According to what I read, the ideal number is 20, however 21 fluctuates, it's like having one more because of what might happen, then it says that the 20 witnesses can be replaced by any circumstance, so there would be 40 witnesses, the main ones and those who can replace, really those who bring this have more knowledge of the subject, I can only say that it seems ideal to me that they have the rest of the backup, it is the same as having a plan B