Υπάρχει is the existential quantifier, and is the most correct. My understanding is that "Είναι δεκαπέντε εστιατόρια" would mean "Those/they are fifteen restaurants," as in "Τι είναι αυτά τα κτίρια; Είναι δεκαπέντε εστιατόρια." This means something quite different.

(Greek speakers: please correct me if this is wrong!)

If you're familiar with Spanish, υπάρχει/υπάρχουν are Spanish's hay.