All private keys are generated from a single private key (i.e., recovery seed). Because it relies on the BIP32 protocol, KeepKey will deterministically generate an unlimited number of private keys derived from that original seed.
KeepKey supports BIP32 wallets, also known as HD ("Hierarchical Deterministic") wallets. This allows KeepKey to generate and store an unlimited number of private keys. KeepKey also supports BIP44, which allows for the organization of accounts. Therefore, KeepKey effectively supports an unlimited number of accounts each with an unlimited number of private keys.