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.
(One-time effect irrelevant)
In the end phase when Old World cards are resolved, the player with the smallest Threat score places one Skaven token in any region where there is at least one Warpstone token, unless there are no Warpstone tokens in play.