on the next button run a backend workflow to create the guest positions

run workflow on list of numbers (can use the list of numbers plugin)
or run recursive workflow that reschedules itself up to the guest number

alternatively change the form design to have the user add/remove each guest individually and then just show the total guest count

data structure is likely
reservation
reservation_guest

if events are large with hundreds of guests then I’d store the event on the guest and do a search for guests.
if events are small with under a hundred guests I would store the guests on the event

if the guests are not accessed regularly then I may not bother with storing a list of them on the event

to show the list you’d either do
search for reservation_guests where reservation is current reservation
or
this reservations guests

depending on how you structured it