How many websites can you have in one Optimizely DXP CMS cloud solution?

There are multiple consideration here to dive into

• License
• Content editing
• Performance
• Team productivity and architecture evolution

License

From a license perspective it's possible to have multiple sites in DXP without an addditional cost. The cost is however depending on page views. For minor sites that isn't used much it can be a good solution to share a DXP instance to reduce costs.
https://world.optimizely.com/documentation/developer-guides/digital-experience-platform/development-considerations/

Content editing

Optimizely CMS has good support for editing experience with multiple sites. It's possible to restrict access rights easily so that editors can only work with content they should work with. It's possible to share common content across multiple sites if needed using blocks for instance. It's also possible to build news feeds etc that combines content from multiple sites easily. Content editing across multiple sites will not be an issue normally even with hundreds of editors.

Performance

Performance it's good to understand that the sites are hosted together using the same resources in the background. They use the same SQL server and the same app service in Azure. With good coding there are really no technical limits to performance if you use caching and CDN correctly. But the larger the codebase is, the more difficult it will become to keep it optimized for performance. This is true for both frontend and backend.
A single site with a couple of integrations and a simple frontend is easy. 10 sites with 100s of backend integrations and shared frontend becomes very tricky.
Generally it's not the amount of content that is a limit but the additional complexity of the code as the codebase grows that will be the limit. That additional complexity will start hurting the performance and later also the availability of the site.

From a technical perspective it's advised to keep this is mind when expanding the functionality of the site or even the number of sites. Is the solution starting to become too complex? A good measurement for this will be given below.

Team productivity and architecture

First it's good to be aware of that organization and software architecture are two sides of the same coin. This is known as Conways law:

Any organization that designs a system (defined broadly) will produce a design whose structure is a copy of the organization's communication structure.