at first she wanted 3, but the time past and she decide to have 5, cause she thought “3 will be ok but 5 will be perfect”
Twilight loves kids too but, she always thought she’ll have 1 or 2. it never comes to his mind that Pinkies wanted 5 kids. maybe 2 or 3 but not 5.
So when she listen Pinkie say she wanted 5 it caught her off guard
Edited
Pinkie: I was thinking 85, but that would be a reference that not many people get.