I'd err on the side of too many - for her ease in terms of not stressing about washing etc
If you can't make that many in terms of time and fabric, then add to her 'stash' with some prefolds or little flats (which she can later use as boosters)
also think about whether they'll be sized or one size fits most (OSFM)
Plenty of Mums with Twins here who I'm sure will be happy to share
