RFC 6238にはTOTPの桁数は規定されていないし参考の実装は8桁を生成しているように見える
TOTP: Time-Based One-Time Password Algorithm
https://datatracker.ietf.org/doc/html/rfc6238
IETF DatatrackerRFC 6238: TOTP: Time-Based One-Time Password AlgorithmThis document describes an extension of the One-Time Password (OTP) algorithm, namely the HMAC-based One-Time Password (HOTP) algorithm, as defined in RFC 4226, to support the time-based moving factor. The HOTP algorithm specifies an event-based OTP algorithm, where the moving factor is an event counter. The present work bases the moving factor on a time value. A time-based variant of the OTP algorithm provides short-lived OTP values, which are desirable for enhanced security. The proposed algorithm can be used across a wide range of network applications, from remote Virtual Private Network (VPN) access and Wi-Fi network logon to transaction-oriented Web applications. The authors believe that a common and shared algorithm will facilitate adoption of two-factor authentication on the Internet by enabling interoperability across commercial and open-source implementations. This document is not an Internet Standards Track specification; it is published for informational purposes.