Quick answer
Decide the total first, then give each set of parents a fixed number of spots to fill as they like. A clear allocation turns an argument about people into a simple question of numbers, and lets parents make their own hard choices.
Set the total before anyone writes names
Parents' lists grow when there is no ceiling. Agree your total guest count with each other first, then decide how many spots go to each family. Do this before anyone starts suggesting names.
Choose a fair way to divide spots
Common approaches are an even split between the couple and each family, or a split that reflects who is paying. Neither is wrong. What matters is that the rule is clear and that each family knows its number.
Let parents choose within their number
Once a family has its spots, let them fill them. If your mother wants her book club and not your second cousins, that is her trade-off to make. Stepping in on every name defeats the point of the allocation.
When a parent is contributing money
A parent paying for part of the day may expect more spots. Talk about this openly when you discuss the contribution, not after the invitations are printed. If the extra guests would push you past your budget or venue, say so and agree the number together.
Say no kindly and once
When a list comes back too long, give the number again rather than debating each name: we have room for twenty from your side, which twenty matter most? Have the conversation together as a couple, and each of you handle your own parents.
Keep one master list
Merge every list into one place with a note of whose guest each person is. It makes duplicates obvious, helps with seating later, and shows everyone where the numbers stand.
Frequently asked questions
How many guests should our parents get to invite?
There is no fixed rule. Many couples split the total evenly between themselves and each family, or by who is contributing. Agree the split before anyone writes names.
Do parents who pay get to invite more people?
Often they expect to. Discuss it when the contribution is offered, and agree the number together so it fits your budget and venue.
How do we tell parents their list is too long?
Repeat the number rather than arguing about individual names, and ask which people matter most to them. Each of you talks to your own parents.