Thanks for posting to PX Community @Stuart !
1 PX User based on that unique IdentifyId.
Gainsight PX leverages a unified User/Account model, so there is only one primary PX Account and PX User record that includes all associated default/custom PX User/Account attributes across all PX Products, Environments, and Channels within a PX Subscription. These PX User/Account records are created and updated using their unique identifiers that you have passed to PX, which is their primary id/key.
You will notice that on the PX User Profile their Account is referenced as “Last associated Account id”, so many of our customer do exactly what you are trying to do and treat PX Accounts as tenants/instances/organizations that the same user can be associated to over time.
If you let the user “bounce around” to different PX Accounts based on their latest PX Identify call content, all user usage stays with the user and account usage stays with the accounts. Therefore, you can successfully analyze usage at an account level or user level. However, the PX User will only be associated to the last associated PX Account if you are trying to filter by Account in PX Analytics.
I hope this helps!