Learn Italian: Purtroppo ce ne sono molti in questo lotto difettoso. - Unfortunately, there are many of them in this defective batch.

While ci sono means "there are", this sentence wants to say "there are many of them". The pronoun ne handles the "of them" part. When you combine the existential ci (there) with ne, you get the cluster ce ne.

Why does ci change to ce in this sentence?

This is a strict pronunciation rule in Italian. Whenever the pronoun ci comes directly before another pronoun starting with l or n (like lo, la, li, le, or ne), its vowel changes from i to e to make it easier to say. So, ci + ne becomes ce ne.

Does molti change its ending based on what ne refers to?

Yes! Even though the specific noun is hidden inside ne ("of them"), the quantity word molti (many) still has to agree with it. Because it ends in -i, we know the defective things being counted are masculine and plural.

Why are ce and ne placed before the verb sono?

These small pronouns (clitics) must go immediately before a standard conjugated verb like sono. You only attach them to the end of the word if the verb is an infinitive, a gerund, or a command.