Maximum output of HKDF
The former. Maximum output must be &lt;= 255*HashLen. So for SHA256 which has a digest of 32 bytes, max HKDF output would be &lt;= 8160 bytes. For SHA512 which has a digest of 64 bytes, max output must be &lt;= 16320 bytes. I can't imagine you would need a key longer than 8160 bytes. If you require multiple keys you can use the 'info' parameter for different contexts/keys from the same PRK.