One Ledger device is capable to have not more than two wallets with individual pins assigned for each. Those PINs with passphrases assigned and SEED are kept in BOLOS, operating system that in turn is in non-volatile memory of secure element.
However there is an option to create pin-less wallets (or as it referred by ledger itself temporary passphrase wallets) in a sense one need to enter relevant password every single time to have the access to that wallet. The number of such wallets is not limited because pertaining passphrases are not kept by BOLOS forever and thus, there is no need for extra memory.