The templates generated by the portal are usually quite verbose and will be separate by service etc. I generally would not recommend using them as is, but use them as a starting point for building your own templates. You can certainly deploy all your services from a single template, you do not need separate templates for each service. In most scenarios you can deploy everything from a single template. You can take what the portal generates and paste them into a single file, but as I mentioned you are likely going to want to try and adjust what the portal creates to remove duplication and verbosity.
You may find that as you add more resources and do more complex things that you might want to break your deployment into multiple templates and have them call each other via the modules feature, but you do not have to.
If you are looking for some training on getting started with Bicep you might be interested in the free course I created at bicepforreal.com