According to the FAQ, Up from Skavenblight will apply for each region containing a Warpstone, meaning you'll get 1 Skaven token per Warpstone. Because the wording is fairly similar, there would seem to be a similar thing for Skavengers, but there are some significant differences. Skavengers specifies exactly which regions get hit (no "adjacent to" phrases") and mentions the player with the lowest threat. Now, if there's no choice here, why would it matter who does it? It would seem to be that only one Skaven gets made for that. The conclusion I draw is that Skavengers gets 1 Skaven and Up from Skavenblight gets 1 per Warpstone. I'm looking for confirmation that this is the case. For reference, here is the text of the cards involved. I've emboldened the similar wordings.

Up from Skavenblight

Place one Skaven token in any region , or adjacent to any region, where there is a Warpstone token . If there are no Warpstone tokens in play, do nothing.

Discard this card instead of adding it to the Old World track.

Skavengers

(One-time effect irrelevant)

In the end phase when Old World cards are resolved, the player with the smallest Threat score place s one Skaven token in any region where there is at least one Warpstone token , unless there are no Warpstone tokens in play.

We've always played it the way you surmise. I can't quite tell if you're actually asking why Skavengers mentions the player with the lowest threat or if that's meant to be a rhetorical question; if the former, it's because there are likely to be several regions with warpstone in them to choose from.

So, these cards allow you to put more than one skaven token in play? Wowzers! I've been playing them wrong all along. Last I checked, 'any' and 'each' do not mean the same thing, but if that's how they want them to work... Holy Buckets!

Well, Up from Skavenblight is definitely multiple. It's explicitly mentioned in the FAQ. The issue is with Skavengers. Kushiel and I believe it's singular with that one.