Learn Italian: Questo canale ha molti ascoltatori. - This channel has many listeners.

Because you are talking about a quantity (molti), you need the partitive pronoun ne (meaning 'of them'). The dislocated sentence becomes: Questo canale ne ha molti, di ascoltatori. Notice that when we use ne to anticipate a quantity, we add the preposition di before the dislocated noun at the end.

Since canale ends in -e, how do we know it takes questo and not questa?

Italian nouns ending in -e can be either masculine or feminine, and you generally have to memorize their gender. Canale is masculine, so it takes the masculine singular demonstrative adjective questo.

Why does molti end in -i instead of being just molto?

When molto is used as an adjective meaning 'many' or 'a lot of', it must agree in gender and number with the noun it modifies. Since ascoltatori is masculine plural, the adjective becomes molti. It only stays unchanging as molto when used as an adverb (meaning 'very' or 'a lot' modifying a verb or adjective).

How is the noun ascoltatori related to the verb for 'to listen'?