Related to these questions:
Can a druid wild shape into a cranium rat and use telepathy?
Can a Druid Wild Shape into a Swarm or “Giant”?
The Cranium Rat question establishes that the full stat block is used, and the Swarm or Giant question clarifies that you can only turn into one creature, per the spell.
Could 18 druids (I'd say 18, because one cranium rat has 2 HP and the Swarm has 36 HP per the stat block in DnD Beyond) wild-shape into 18 Cranium Rats and form a swarm, per the description of the Cranium Rat, and gain all the additional benefits that a Cranium Rat Swarm has?
The description says:
Evil Collectives: Cranium rats are no smarter than ordinary rats and
behave as such. However, if enough cranium rats come together to form a
swarm, they merge their minds into a single intelligence with the
accumulated memories of all the swarms constituents. The rats become
smarter as a result, and they retain their heightened intelligence for
as long as the swarm persists. The swarm also awakens latent psionic
abilities implanted within each cranium rat by its mind flayer
creators, bestowing upon the swarm psionic powers similar to spells.
Also, the Cranium Rat Swarm's Innate Spellcasting (Psionics) says:
The swarm's innate spellcasting ability is Intelligence (spell save DC
13). As long as it has more than half of its hit points remaining, the
swarm can innately cast the following spells, requiring no components:
At will: command, comprehend languages, detect thoughts 1/day each:
confusion, dominate monster
Now, the Druid Wild Shape feature says you can't cast spells, but the description of the Cranium Rat says:
The swarm also awakens latent psionic abilities implanted within each cranium rat by its mind flayer creators, bestowing upon the swarm psionic powers similar to spells.
So this means that the 'spells' are NOT in fact actually spells, but are instead psionic powers, even though the stat block does call them 'spells'.
Does this mean that such a collective of Druids get access to these powers as well if they Wild Shape into Cranium Rats and form a swarm?
Side Note: It would be an interesting story element to have a group of Circle of Decay druids using this ability to combine their knowledge and experience periodically, and expand their power by teaching each other new skills, magic, and other things. Especially if the GM rules that the stats for the Wild Shaped druids make a much more powerful 'swarm' by scaling the swarm Cha, Int, and Wis stats based on the druid's stats (Since Wild Shape lets you keep those stats when in Wild Shape) the same way they are scaled up from the Cranium Rat to the Cranium Rat swarm.
Best Answer
It's up to your DM.
We don't have any rules about what has to be true for a cranium rat swarm to form.
We don't know if there are checks involved, we don't know how long it takes, we don't know if the cranium rats have to be mentally compatible, and in particular we don't know how many cranium rats it takes to form a swarm. (You've suggested that 18 cranium rats should be correct, and you derived that number by dividing the hit points of the swarm by the hit points of a rat, but that is your house rule and not necessarily correct.)
It would be an error for us to say "yes you can do this", because someone might then point their DM to the answer and say: "look, rpg.stackexchange says you have to allow this!" and we shouldn't be issuing house rules for you.
I do think this would be a cool thing to have happen, but it would have to be done with the DM's cooperation anyway since you're unlikely to have 18 player character druids in the same campaign.