Rule reminder
C'è and ci sono can be combined with question words: Dove c'è...? (Where is there...?), Quanti/Quante ci sono? (How many are there?), Quando c'è...? (When is there...?), Come mai non c'è...? (How come there isn't...?)
Quante persone _____ alla conferenza? (How many people are there at the conference?)