Two official services

Number starts with	Use	Note
GB	GOV.UK: Check an EORI number	The UK service checks only numbers beginning with GB. If the business agreed to share it, you can also see its registered name and address.
XI or an EU country code	European Commission EORI validation	The UK service sends numbers that do not start with GB to the Commission's tool.

Checking a list

The GOV.UK checker verifies GB numbers and the European Commission checker verifies XI numbers and numbers from the 27 EU member states (GOV.UK). The format checker has a list mode: paste several numbers, see which have a valid shape and which official service to use for each, and download the result as CSV. It never sends the numbers anywhere. The sources we read do not say how many numbers the official services accept at once.

If you cannot find your own GB EORI number

GOV.UK says to sign in to the digital EORI registration at gov.uk/eori with the details you registered with, and not through any other login page; your number is shown once you are logged in. You can also find it in the financial dashboard of the Customs Declaration Service (CDS) (GOV.UK).

What a format check cannot tell you

A number can have the correct shape and still not be registered, or be registered to a different business. Only the official services can confirm that. The format checker on this site tests only the shape: country code and length.

Tips

Remove spaces before pasting if a service rejects the number; HMRC's own field accepts spaces and either case (HMRC pattern).
If the official service says a number "did not match any registered business", ask the business to confirm the number and the country that issued it.