The swarm rule appears in Numenera on page 232, and in The Strange on page 260. But I can't find it in The Cypher System Rulebook.
Where can I find it? If is not present in the book, why is that?
Page 296 mentions in an aside
GM Intrusion: A swarm of twelve ordinary rats—each level 1, but acting like a level 3 swarm—is summoned by the high- pitched squeaking of a giant rat.
But otherwise no, the text is not to be found (I searched my PDF for "swarm" and other relevant phrases, like "group of"). You'd have to ask @Montecookgames why, but given that aside, it seems like it got cut by oversight.
It's in the Ninth World Bestiary, on page 13.
The GM can take any creature and have a group of six to ten of them attack en masse as a single creature that is two levels higher, inflicting double the original creature’s normal damage. So thirty level 3 abhumans might attack as five level 5 mobs.
Although not using the term 'swarm', the rules for multiple enemies working together can be found on page 349 of the Numenera Core Book:
As a general rule, for every four creatures working together, treat them as one creature with a level equal to the highest of them plus 1, with a minimum of a +2 damage bonus. So a level 4 bandit who has three level 3 allies could team up and attack one foe as a level 5 NPC.