I think my party was around 10-12 for the Sepulcher. That's one of the best dungeons in the game for me so far but it is a bit tricky to navigate at first. A couple of the key doors are awkward to spot so definitely search all sections carefully.
The last crawl in the Valley of the Dead to finish the Varnhold Vanishing quest is also significant but providing your party is reasonably well equipped and with enough rations (and some means of death ward and restoration) to not have to repeatedly retreat to the capital, I think you'll be fine. I managed to complete that dungeon without resting, but that'll naturally depend on many factors. (Party composition, spell availability etc)
Party wise, I'm using a very melee heavy group - dwarven fighter PC, Valerie, Amiri, Harrim with Ekundayo and Octavia at the rear. I rely on Harrim and Octavia for buffs and spells with lots of scrolls and potion use. Ekun with the Devourer of Metal bow is a real powerhouse and overall is quite reminiscent of Kivan (stoic ranger) from Baldurs Gate. His dog companion is also quite useful and fun to keep alive.
I've got Harrim outfitted with the mallet of woe and often tank with him or place him just behind for spell buffs. He's in essence another take on the Xan persona from BG.
I haven't taken Tristian much but he is my only counsellor. I don't mind Linzi; but she's a bit too Alora/Imoen and gets on my nerves at times. I didn't really take to Jaethal or Regongar. Played with Nok-Nok a bit for the amusement factor. Jubilost looks to be powerful (as is alchemy generally) but I can't stomach his smugness. Found him very late on. I sense he'd be fun to use in an evil playthrough though.
Best of luck finishing up the Sepulcher.