And also one scenario: Our parents might have some relatives that they would like to invite to your wedding but you might not have seen them more than 3/4 of the time growing up. How would you manage a situation like this? p/s: Asking for a friend, seriously haha
Talk to your parents, set limits, and try to reach a compromise.
I put my foot down on my parents inviting their colleagues - to me, the priority for guests was first direct family (no distant cousins whom I've not seen or hardly see, please), then friends and our colleagues, followed by my parents friends, then their colleagues.
Some try the route of making their parents pay, but I don't think that's very nice la cos our parents don't earn a lot, so we didn't impose that. To each their own.
We allocated a limited number per category outside of direct family. So if my parents wanted to invite more people, sure, but only if we have enough seats left.
As it turned out, there was enough to go around for everyone in the end cos we opted for a caterer instead of a hotel banquet :)