Welcome to the HP Support Community. I'd be happy to assist you with the Instant Ink query.
You can have multiple printers enrolled in Instant Ink with one email address as long as the printer is eligible for Instant Ink. Go to the Instant Ink page to and scroll down to check if your printer is eligible.
HP OfficeJet 7500A is not eligible for Instant Ink due to unique supplies that:
1. Have different fill levels from store purchased supplies
2. Can be enabled or disabled from use in the printer via the Instant Ink program (the cloud).
This is also due to unique firmware in the printer that enables specific type of communication to the cloud.
Hope this helps! Keep me posted.
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