One way to do this would be to use WordPress Multisite. This allows you to have multiple sites within one single installation (a so called network).
All sites can use the same theme/design and you can choose which users are allowed to edit content on which sites.
In case you’re not sure if that really works, rest assured: this is how WordCamp.org organizes all their conference websites under the hood. And they have lots of them.
There’s also tons of documentation on the web about Multisite as it’s really a proven way to organize sites.