From my experience you cannot have multiple email addresses per one device registration. It sounds like from your testing that you can have multiple devices per email address, which should be allowed, but I would not feel comfortable putting 10,000 on one email address.
It’s easy enough to hide the email/login process by creating a gmail account for your app, then use email addresses like:
yourappname+somerandomvalueforuser@gmail.com and keep it 1:1