I don't ever post here (only lurk), but this is actually a really complicated question, and I thought I'd give it a go! I know quite a bit about the show (as only an audience member, but still), and I still don't think I can adequately answer it. I'll tell you what I do know: the problem is that the tracks are different for pretty much all the productions, from what I can tell, as are the understudy assignments. You can tell a lot about which tracks were used for principal singers by checking out the credits for the various cast recordings. So if the same actor played Combeferre and the Bishop, he'll often be credited for both in the liner notes of the recording. Most of these liner notes are available online, I think, at least for the major English-language recordings. This doesn't help with non-singing ensemble roles, but there you have it.
The tracks can vary widely for the male ensemble roles. For instance, Combeferre, as mentioned, does often double as the Bishop (in both the OBC and the current B-way cast, if I'm not mistaken), but I remember back in the US Third Nat'l Tour in the early 2000s, they had Combeferre doubling instead as the Factory Foreman, plus Grantaire doubling as Bamatabois. Courfeyrac now doubles as the Factory Foreman in the current B-way production, I believe, and several of the students used to also be in Thenardier's inn, either as customers or as drinkers. These are just a few examples of possible variations. There was a backstage video they made for the London production a few years back in which Feuilly and Bamatabois describe their tracks in some detail, which gives you an idea of how these male ensemble tracks are arranged there: https://www.youtube.com/watch?v=28VWo9q8vSU (The relevant bit of that video starts around 2:30.)
Tons of costume changes! No matter how you set up the tracks, it'll be quick-change madness. Just the transition between "Turning" and the wedding scene is an insane turnaround (no pun intended): first for the ladies (from street women to wedding guests, presumably doing the change during "Empty Chairs at Empty Tables"), then for the men (from students in "Empty Chairs at Empty Tables" to wedding guests, during...when??? The last verse of "Empty Chairs" and the beginning of the wedding scene change??? Wild!). The male ensemble playing students doesn't have it so bad, since they're stuck on the barricade for most of the second act, but the male ensemble playing Thenardier's gang seems to have it a little tougher, with more changes. Main singing roles like Marius, Enjolras, Eponine, and Cosette I have noticed early on in the first act, the men playing convicts and guards in the first scene and the women prostitutes in the "Lovely Ladies" scene.
If it would be any help, I might be able to dig out one of my old fancy playbills from the Third Nat'l Tour to see how the roles are credited there. This information should be available for the currently running productions through their cast listings in their playbills and on their website, yeah?
Sorry about the long-winded answer without much answer in it! I think a lot of how you break down the roles depends on which voice parts you need in which scenes, and who's available when, some of which can be ironed out in rehearsals. Good luck with those quick changes, though, and be sure to name-label all those waistcoats real well!
