Shops
The Reaction Commerce API is a multi-shop system, which means that more than one shop can exist within the same installation.
While the Reaction Commerce API has multi-shop concepts built in, the default UIs for Reaction Commerce (example-storefront and reaction-admin) have only been built to support a single-shop setup out of the box. Any multi-shop UI needs will require additional development work.
Technically speaking, the current state of multi-shop support is that most entities in the database have a mandatory shopId field in their schemas, and most API calls require you to specify a shop or shops. There is a Shops collection, which can list as many shops as you'd want, with different users having different rights on them thanks to the Groups collection. This architecture can be leveraged to build custom, multi-shop projects.
Disclaimer: multi-vendor marketplaces
It is crucial to make the distinction between multi-shop projects and multi-vendor marketplace projects. Multi-vendor marketplaces are typically projects where vendors can sign up on their own, create their shop and have their products listed on a single, unified catalog (i.e. similar to eBay or Amazon).
At this stage, there is no plan to support multi-vendor marketplaces out of the box on Reaction Commerce. However, you should check out the community plugins that may exist to offer this feature.