MAX_ENABLED_ROLES specifies the maximum number of database roles that a user can enable, including sub-roles.
The actual number of roles a user can enable is 2 plus the value of MAX_ENABLED_ROLES, because each user has two additional roles, PUBLIC, and the user's own role. For example, if MAX_ENABLED_ROLES is set to 5, user SCOTT can have 7 roles enabled, the five enabled by MAX_ENABLED_ROLES plus PUBLIC and SCOTT.