Since there's no specific mention of the number of players, lets look at the challenges in the pdf. We'll go over each plot point, and see what dice/rolls are needed to succeed. There's a quick tl;dr at the end if you just want my opinion on party size. Potential spoilers below!
- Prequel: if players were drinking, they get +2 vigor and -2 smarts.
It will prove advantageous for most players to be drunk (unless they rely on smarts!)
- 1) Common Knowledge at -2.
A d8 in Common Knowledge gives slightly better than 50% chance of success. However, a raise on this roll informs the PC that a legion of undead lie below. At this point, my PCs would probably attempt to set the entire catacombs on fire; ymmv. Regardless, they should be able to grab some quick supplies from above. I would probably have the NPC soldier join them upon learning.
- 2) Strength roll to jump 2" (or build a bridge)
A piece of rope, a ladder, even a broken door can circumvent this challenge if no-one has the strength for it.
- 3) Fight two rat swarms
Assuming your party has torches, this is pretty trivial. Otherwise, stomping them works well.
- 4a) Fight eight zombies with a guaranteed surprise round.
How the group uses the surprise round will largely determine how difficult this is. Setting the zombies on fire will mean your group wins unharmed; charging in blindly might result in a few wounds. Since the backstory explains that the PCs have already encountered zombies, they should be familiar with the headshot weakness.
- 4b) Vigor roll at -2 (zombie poison on death)
Good thing you drank that alcohol, right? A d6 vigor gives a 50% chance of passing this drunkenly, and I would house-rule that a PC can only be poisoned by the gas once.
- 5) Fight Bone golem with N zombies
The first real challenge. A notice of 4 spots the golem's weakpoint, and a raise prevents the golem's surprise round. Once the weak point is spotted, a called shot at -4 will seriously wound the golem. A shooting skill of d10 gives a 50% chance of hitting the call shot. The GM can provide additional zombies (if any) based on the parties success this far.
- 6) Fight Necromancer (novice)
The necromancer's main strength is his bolt and zombie powers. If you've prevented his ritual, he's a slightly buffed novice spell caster. If he completes the ritual, then he has 50 PP and gets more if he kills a PC. He's easiest to kill with arrows or a greatsword to the head, as his advantage is in firing long-range bolts.
So how does all this help? For one, we can see that most of the challenges require abilities easily obtained by novice characters. A d10 shooting skill is the highest required die for a 50% success chance, and passing that roll will make short work of the bone golem.
The combination of skills required isn't really varied either. One character focused on smarts (knowledge, notice, spellcasting), one character focused on agility (parry, shooting), and one character focused on vigor (tanking, melee fighting) should be able to pass all the challenges. A specialized gun-toting holy warrior could likely solo the entire challenge.
My advice, based on these stats and a little gut intuition, is to have a minimum of 3 party members and a max of 5. If the PCs make it to the final chamber with full resources, add some zombies to increase the challenge.
A lot of the difficulty will also depend on the experience of your players. Experienced Savage World players know the best way to position themselves on the board, take advantage of movement, and minimize area of effect damage against them. Newer players who are still learning these concepts will struggle more.
tl;dr: For this specific challenge, a party size of 3-5 seems ideal. Less characters will have trouble with some of the challenges, more will make them trivially easy.