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Sandbox Accounts are defined at the ACCOUNT-LEVEL in the RingCentral Connect Platform currently. We do not support creating multiple USER-LEVEL Sandbox Accounts within a single RingCentral Account at this time. However once the Sandbox Account is created and associated with your RingCentral account, you are able to use the Developer Portal to define as many new applications as you need.
I have highlighted the location of where you create / view your Sandbox Account in the screenshot below.