Well, that depends on the purging characteristics of Fastly - this blog post is quite nice to read https://www.fastly.com/blog/building-fast-and-reliable-purgi...

We have a limit - you can only "tag" up to 1k items within one query. If you have more items in there, we can't purge all of them.

Due to the nature of the broadcast protocol of Fastlys purging implementation, I assume that it scales quite well - they have really big customers using this for a few years already.