There is no limit for adding containers.
You do not need to have a different UserGuiding account for each website. You can create multiple containers and prepare different guides to show on different websites.
When you create a separate container for each website, the user is able to see the guides which are assigned to the same container in the UserGuiding Assistant.
So it will be just like this;
Site A > use the Embed Code for Container A
Guides Prepared for Site A > assign to Container A
Guides that are available in the UG Assistant in the Site A > are the guides which are assigned to Container A.
And you can follow this process for each different website.
Check out this video to learn about the basics of Containers & Integration