Yes, multi-session app streaming is supported so users can share the same VM resources when using RemoteApps.
There are different licensing options with regards to licensing external users who are not members of the same organisation, as explained in the link below.
https://learn.microsoft.com/en-us/azure/virtual-desktop/remote-app-streaming/licensing
Just to note on external users, there is currently no support for external (i.e. B2B / guest users) for Azure Virtual Desktop, i.e. all users must exist on the same tenant.