The Hyperlane v1 system runs as PoA (Proof of Authority) network, and as of the time of writing, there were 6 signers. However migration to v2 is on the roadmap which has built-in cryptoeconomic incentives to mitigate collusion via staking and slashing of validators which signing incorrect Merkle roots.
For the initial implementation, please refer to this audit document. When referring to the audit document please be aware, that Hyperlane used to be named Abacus. This name is used in this audit document.