@Guillermo Mateo
Thank you for your post!
Unfortunately, creating a sub-tenant, or a tenant within a current Azure Active Directory tenant isn't possible since an Azure tenant represents a single organization. However, if you'd like this to be a implemented, since your organization may benefit from this, I'd recommend creating a feature request using our User Voice forum, so our engineering team can look into this.
Additional Links:
Create a new tenant in Azure Active Directory
Create an Azure Active Directory B2C tenant
If you have any other questions, please let me know.
Thank you for your time and patience throughout this issue.
----------
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.