Well, I guess it partly depends on how long this campaign is meant to be. We played a multi-year Ravenloft campaign. The big bad we eventually faced was the dark lord of the domain, but we played against all sorts of villains and monsters to get there.
That was a single domain adventure, because it was a Castlevania adaptation, so it was entirely standard. From what I always gathered from previous editions, there were basically two standard assumed ways of using Ravenloft.
1) Single adventure. The Mists snag your characters from their usual adventures in some other setting, you have an adventure or two in Ravenloft, and then you return home to continue your campaign.
2) Full campaign. You characters are either from Ravenloft, or snagged by the Mists, but you a going to play their entire adventuring life there. If they were from elsewhere, maybe they escape at the end to retire somewhere else. During this campaign you typically travel to various realms.
In the second scenario I think confonting (with an assumption of possibly defeating as a big bad) a single Dark lord would be a fairly common element. Other dark lords may or may not make cameo appearances. What I would not consider to be assumed is facing the dark lord of each domain you visit. Even facing two dark lords over the course of a campaign would be akin to facing two gods or demon lords or something. They are mostly part of the setting, and it's rarely assumed that the PCs are there to blow up the setting.
In the first situation, I think it could go either way. The assumption is probably that most of the times you won't face a dark lord. But then again, this is essentially what the original Ravenloft module was, and you did face the original dark lord, so I wouldn't see that as uncommon.
I know the OP didn't want to get into edtion differences, and I'm trying to avoid doing so, but I think there are certain changes assumptions with regards to 5e that need to be noted.
In 5e WotC has intentionally created the assumption that you create a party of characters, run them through one of their campaign length mega-adventure books in the space of about a year, retire those characters, buy a new mega-adventure, create a new party, and repeat.
This play loop is 5e-only thing.
4e (and Pathfinder) had some similar premade campaign stuff, but I believe their campaigns were typically assumed to run longer than a year.
Prior to that, this was the assumed campaign progression:
You pick the setting and create a party. You play a combination of pre-made (whether individually purchased, or from magazines or downloads) and DM made adventures, that probably have no relationship to each other, and you do this for years. There might develop ongoing villains, or even a (often unknown at first) background campaign big bad to eventually be addressed. You end the campaign after accomplishing some major goal (unless it trails off early). It's an-ongoing episodic following of the adventure of your party. Think Stargate SG-1.
Now, there have always been variant ways to play. You might have played an endless campaign where you retire characters and bring in others but the same account continues. West Marches style is a variant of that. One-shots have always been a thing, though I personally think actually doing them in a single short session rather than 2-4 sessions wasn't very common until Adventurer's League, which sets 5e's other main style. And short campaigns where you make a group you are only going to play for 10-20 sessions through a specific arc wasn't terribly uncommon.
But I think it is sort of necessary to interpret answers to the OP in light of the fundamental restructuring of campaign style that 5e intentionally introduced.