You can get a general idea of where the "sidequests" and "branching quests" are in each campaign just by starting to play them: The game is quite open from the very beginning in that regard.
By "sidequest" I mean a scenario that you can play if you want to, but you can progress through the campaign without playing it if you prefer. "Branching quest" means you have a choice of two different scenarios, and you must choose one of them to the exclusion of the other.
For example, from the very beginning of Roland's campaign, it is obvious that scenario 3 is a sidequest, and that scenarios 7 and 8 are branching quests.
You always get at least one reward for completing a sidequest or a branching quest, and it's generally easy to guess what that reward is going to be when you read the summary of the scenario.
In terms of number of scenarios, Archibald's campaign has 11 (of which you can play 8 or 9); Roland's campaign has 10 (of which you can play 8 or 9); The Price of Loyalty has 8 (of which you can play as few as 5 or as many as 7); Descendants has 8 (of which you can play 6); Wizard's Isle has 4 (of which you can play 3 or 4), and Voyage Home has 4 (of which you can play 3).
The reason why you generally can't play all of the scenarios is that some of them are "branching quests", which, as I say, are mutually exclusive.
One more thing about Archibald's and Roland's campaigns in particular: In scenario 5 of each of those campaigns, you have the option to defect to the other side. This makes scenario 5 easier in some ways (since instead of controlling one castle and needing to take over three enemy castles, you control three castles and must take over one enemy castle) but makes the rest of the campaign harder in others (since you lose any rewards that you may have earned before scenario 5).
I see. So if I have a choice between scenario A and scenario B, then finish either, will I get the same choices (say, C and D), or do they branch out and I'd get choices AC, AD, BC and BD?
I'm not entirely sure what you're asking, so I'll give you a few specific examples from Roland's campaign: Whether you complete scenario 3 or not, the map of scenario 4 will still be the same. (The only difference will be whether you have the reward from scenario 3, and that makes no difference to the map layout.) And whether you complete scenario 7 or scenario 8, you'll always have to play the same scenario 9.
So in short, the answer is neither: If you have a choice between scenario A and scenario B, the next scenario after that choice will usually (i.e. with one exception, in Price of Loyalty) be scenario C. And that one exception boils down to scenario A actually being two scenarios, so it's not that complicated either.