I found online a checklist stating the following rule for multiple enemies / swarms:
4 creatures = 1 creature of the highest level + 1, minimum damage bonus +2
This is different from the official for swarms (explained in this answer).
I can not find this rule in the Numenera Rulebook or the Cypher System Rulebook. Is it a rule of the house or a failure of my intellectual work to find it?