Interesting. There is technically no limit to the number of job roles a user can have:
"A user can have multiple job roles in Workfront. Job roles are essential in resource management. There is no limit for how many job roles a user can fulfill. However, we recommend to not assign one user to an excessively large number of job roles, because resource management might become too complex for these users." - here
Are any of the job roles you're trying to include actually deactivated roles? Maybe throwing an error if you're using inactive roles...
If this helped you, please mark correct to help others : )