How to create SaaS application with Python and Django
Software as a Service is just a marketing word, it's technically no different from a server that is accessible over the internet. So question 3 makes no sense. That leaves us with question 1 and 2:

What do you mean with 'app' in this context? Your web application (built with Python and Django) can have multiple Django apps (components that make up the web application) but I think that's not what you mean. You can build your website in Python/Django and have various customization options depending on which user (client) is logged in. For example, a premium client can have several advanced options enabled but it's still part of the same codebase. It's just that some options (buttons/controls, etc) are not shown for certain clients
Django has plenty of tools for user management, permissions and groups. You can give each user (each client) different permissions and these permissions determine what they can do. Database access should be managed by your web application. For example, the code determines what information needs to be displayed on the webpage (depending on which client is logged in) and that code retrieves the information from the database. Depending on the scale that you're aiming for, you can also specify which database should be used to retrieve the information from.
Can you advice me with some articles/applications that allows you create SaaS(Software as a Service) application with Python and Django.
For the moment the general topics I do not understand are:

Do you have one working application for all clients or one app per client
How do you manage database access, permissions or different DB for each client
Are there any tools that allows you to convert one app to SaaS