I assume you have 4 PCs? The first thing you want is to decide how you're going to introduce the cultists. The last thing you want is for them all to run out of a single door, roll initiative lower that the wizard, and be reduced to a melted mass of flesh courtesy of a fireball.
Assuming you can get them to rush the PCs from different angles, consider things like surprise, distance, etc in your planning. Basically, are the PCs going to get to act or are they going to begin combat trying to resist grapples?
Once the cultists reach the PCs, it becomes a matter of probability. Assume 7 cultists per PC (minus any killed by PCs during the bumrush).
Maths break. This is actually fucking terrifying. If you have the PCs roll once to contest all 7 checks by the cultists, then there's a 52% chance for DC 19 or 20, and 90+% for DC 15+. Your PCs are going to need to be very lucky to avoid being grappled. Really though, they should roll separate checks against all 7 grapples. In the best case scenario, +3 prof and +5 ability (ignoring expertise for now), the player will still have roughly 19.5% to be successfully grappled per cultist. That's a 22% chance to escape all 7 grapples (0.8057 = ~0.219). For a character with +2 ability and proficiency it's 8%. For a character with +2 and no proficiency it's 2%. For a character with no bonuses it's about 0.5%.
Bottom line: Your PCs are going to get grappled and pinned instantly.
At this point the cultists can start stabbing. Give them daggers, not scimitars, if you want the PCs to have any chance at all. +3 attack roll, with advantage, hits full plate (AC 18, no shield or dex bonuses possible) 51% of the time. A wizard in robes (AC 10) is going to get stabbed 91% of the time. 7 cultists are going to do about 15 damage per turn to him. Meanwhile, he has to make a successful athletics / acrobatics contest against every single cultist to escape. Even your 20 str warrior is going to have a hard time surviving this.
Alternatively, you can say that the cultists need to use their action to maintain the grapple. Not strictly what the rules say, but we'll pretend they need to use both hands to hold down your heroic adventurers. This becomes more interesting. It lowers the damage, and increases the chance of escape by the PCs, but will depend on how many cultists focus on pinning and how many focus on stabbing. Even four cultists pinning is going to be extremely difficult to escape. And once the player escapes, that's their action used. The cultists can just grapple again next round.
What to do? I would try it with 4 cultists per PC, and have two holding and two stabbing. A 20 str, athletic warrior can escape four grapple attempts 42% of the time, and two attempts ~65% of the time. All he has to do is get an action and he can start cleaving cultists to smithereens and then help his friends. A wizard with +2 from dex and no proficiency has ~10% chance to avoid four grapples, and ~33% chance to escape two pinning him.
There's a lot happening here and it gets kinda murky as to what bonuses you want to let players have, etc. Ultimately, 6-8 cultists per PC is too high, I think. Lower that number, or give PCs a chance to lower it with spells / ranged weapons as the cultists charge them.