A lot more than 15PB; it was over 20PB last year. And that figure doesn't count the duplicate copies they keep of everything.
"Today, the Wayback Machine houses some 388B web pages, and its parent ... The Internet Archive’s collection, which spans not just the web, but books, audio 78rpm records, videos, images, and software, amounts to more than 40 petabytes, or 40 million gigabytes, of data. The Wayback Machine makes up about 63% of that."
https://thehustle.co/inside-wayback-machine-internet-archive
It's essentially the only cure for the serious link-rot problems in Wikipedia's references.
"We preserve 750 million Web pages per week! We’ve saved 35 petabytes (that’s 35,000,000,000,000,000 bytes) of data."