Imprecise quantities are expressed using adverbs of quantity, which are usually followed by the preposition de.
Il a beaucoup de chiens. — He has a lot of dogs.
Il boit trop de bière. — He is drinking too much beer.
Combien de jupes as-tu ? — How many skirts do you have?
No matter whether the noun following de is plural or singular, de always remains de.
However, other articles can follow adverbs of quantity when the noun is specific.
Beaucoup des (de + les) amis de mon frère sont là. — Many of my brother’s friends are here.
Je veux plus du (de + le) même. — I want more of the same.
This latter case does not apply to this sentence as the speaker is asking about skirts in general - there is no specificity.