Groups Used to Have More Players
It wasn't unheard of in RPG gaming's early days to play in much larger groups than are expected when playing contemporary RPGs. A group of 6 was reasonable, and I played AD&D in high school with groups as large as 12, with players rotating in and out week by week. Thus it's not uncommon for older adventures to be built for larger groups. As the complexity of characters and plots increased and the popularity of tabletop wargaming and role-playing games in general decreased, larger-sized groups became less and less common.
Parties Used to Have More Members
Exploring a dungeon in AD&D was not a task done lightly. PCs interviewed potential employees, hired or didn't some NPCs, allocated (very small) treasure shares to those they hired, took them into the dungeon, congratulated them if they survived, and then either murdered them or took them on to the next dungeon. The comic Knights of the Dinner Table explores this dynamic in great detail. So, yeah, filling out the ranks with trap-bait--I mean, linkboys--was totally a thing.
...But Not So Much in Tournament Adventures
The Slavelords series originated as a tournament series wherein awards were given depending on time it took the party to reach certain points in the adventure, what goals the party had accomplished, how many encounters the party overcame, and so on. It wasn't designed--like some adventures--for forays into the dungeon, return to home base for recruits, and repeat.
Of the 6-9 characters that they expect to undertake that adventure, half will die. I suggest rethinking letting players play more than 1 character and allow 2 characters; the characters won't have--or, at least, shouldn't have--deep backgrounds anyway. I especially urge this for the players otherwise burdened with low-level wizards, as once the wizard casts his spell he basically hides behind the fighter until tomorrow.
Otherwise, if you want a firmer hand and more opportunity to role-play, encourage them to employ hirelings. Or sheep--they're good at finding pit traps, too.