There's a list of seemingly obvious ones we require purely because we don't understand the family dynamics or have anyones details.
You don't want your tog going, 'Ok can we have the brides mother and father' when in fact there may be a divorce or a death in the family.
I always send a guide, along with an upper limit along with a VIP section for those that are really important.
20 formals maximum with 45 minutes to do them in is realistic. I can do them in 25 if the guests are switched on enough.