Yeah, no doubt a lot of devices were affected.Well clearly enough were that Apple did what they have never done and released an update at 12:15AM on a Saturday to address the issue. They must have some info you don't as far as how many are effected. But the bottom line, that answers your initial question as to why they released it before Apple Pay Cash was turned on.
There are 800+ million iPhones in use; even if only 3% of devices had the issue, that’s about 25 million. If around 10% were affected, it would be closer to 100 million.
I could easily see 5-10% of users having an app that utilizes local notifications, which triggered the restart loop bug that 11.2 was quickly pushed out to fix.