When setting up the DXC Service Episerver Find will be added as default search for your solution, and a Find index will be automatically set up and configured as part of the services. Sites will share the same index in a multisite solution. See the DXC Service section for more information.

You normally have one index per EPiServer install (which can have multiple sites in the tree structure). You normally don't share index if you have 2 separate Episerver environement. (Reindexing etc will be problematic...)

Check out

http://www.episerver.com/digital-experience-cloud/order-episerver-find/ 

for how many indexes you get for a specific license.

When ordering DXC you get 3 environments (integration, preproduction, production), where each one of them can be a multi-site setup, and where each one of the environments have their own Find index.

In a multi-site scenario the sites share the same index but each individual site have their own 'partition' of the index and also a shared 'partition' for shared content.

/Henrik