Clerk refers to the National Institute of Standards and Technology (NIST) guidelines to determine the character rules for passwords:
Verifiers SHALL require subscriber-chosen memorized secrets to be at least 8 characters in length. Verifiers SHOULD permit subscriber-chosen memorized secrets at least 64 characters in length. All printing ASCII [RFC 20] characters as well as the space character SHOULD be acceptable in memorized secrets. Unicode [ISO/ISC 10646] characters SHOULD be accepted as well. To make allowances for likely mistyping, verifiers MAY replace multiple consecutive space characters with a single space character prior to verification, provided that the result is at least 8 characters in length. Truncation of the secret SHALL NOT be performed. For purposes of the above length requirements, each Unicode code point SHALL be counted as a single character.
While these rules might seem lax independently, NIST's additional leaked password protection guidelines do more to prevent the use of unsafe passwords.
Also, please bear in mind, that passwords are not a requirement for using Clerk. Applications can be configured to use a passwordless strategy that relies on your users being sent one-time passwords instead.