Best structure to a SaaS software

[Fury Design]

Hello, I'm new in this community and I began my studies on Ruby On
Rails. I am developing a SaaS software, but I have a doubt as to its
structure.

The SaaS software that use a subdomain like suaempresa.dominio.com to
access the system. All companies use a single application in the same
database or an application and a database for each company?

I think the ideal would be a single application and database for all
companies and not need a subdomain to access the system. But there is a
problem, how this is a single database, how to do the backups?

Thanks.

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