Do you have any sources for this claim?

As far as I am aware Google doesn't publish any statistics about the size of its index, which no doubt varies.

https://www.google.com/search/howsearchworks/how-search-work....

Well what do you know, they contradict the claim made above.

Sorry, they've crawled trillions of pages, and narrowed it down to an index of 100s of billions. Conveniently, the link answers your question of "can you have PB sized indices?" to which we can clearly say, yes.