I don't have a big NAS but it's easy to fill a bunch of hard drives. I was born in the 80s which means that I want to watch a lot of comedies and action movies from the 70s (old stuff), 80s (childhood), and 90s ("teenagehood"). Also a lot of current movies because there is a lot of good stuff (I watched the Dune 2021 with my wife and we both agreed that it's fucking awesome, there are a billion good movies out there). I only have a thousand movies but it's already weighing more than 1 TB (each movie is 1 GB in 1080p because I don't want 4K stuff on my shitty TV). Most of those movies are what I consider "good movies" that I want to watch again in the future, the same way people had VHS libraries in the 80s.

Now add TV shows. Each TV show has approximately 20 episodes for 10 seasons. It's an additional 3 or 4 TB of stuff that I may want to watch in the future. You can easily get a few interesting TV shows whatever your preferences are. Any kind of video that you may want to watch twice in the future. For example, I always wanted to watch the X-files when I was younger but I didn't had the time to do it. All the X-files episodes add 80 GB on my NAS. South Park: 50 GB, Bewitched (1964-1972, yes, I love that old stuff): 80 GB!

Also don't forget cartoons (I have 500 GB of cartoons and anime), it can be big.

Music is a smaller issue because I compress everything from FLAC to the Opus codec at high quality and, while the files may be small, it still counts. Every album weighs 100 MB, a discography is 1 GB. If you like a hundred artists, you have 100 GB of additional data.

As for finding stuff, I don't use Sonarr or stuff like this, I only note what I remember from the past (or new movies) and get it later from TPB.