Since Pinkie pie take care of Ms and Mrs cake babys (Pound cake and Pumpkin cake), she always wanted to have a big family, she even sometimes imaginate herself with her future babies.
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