You can have as many website as your server hardware resources can handle.
The best option for you is to create separate databases for each project you create. You can set different websites in IIS and set them to use one application pool, which is properly configured.
Sitefinity stores all images/videos/documets in the database OR if you later decide you can choose other StorageProvider. However almost everything depends on the purpose of those websites.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items