There are a lot of them, and one cable will hold about 72 individual fiber strands, that is a lot of full optical bandwidth. Space division multiplexing is a way to make more then one link over the same fiber strand, so you can squeeze a lot of bandwidth over one strand of fiber optic.

(https://community.fs.com/blog/application-of-space-division-multiplexing-sdm-in-submarine-optical-cable.html)

Due to the distributed nature of the internet, you don't actually have to cross undersea cables very often. There are caching points and content delivery networks so you are usually not more than a few hops away from your content. This can be arranged through something called 'transit agreements' and 'peering agreements'; that is an entire essay unto itself, but suffice to say almost every modern ISP POP (internet service provider point of presence) has a Netflix caching server in a server rack plugged directly into the service provider equipment. That shortens the distance between the content and the subscriber.

Source: I work for one of the major US ISPs.