g suite – Google Workspace Custom Domain

Has anybody got any experience with Google Workspace?

Im thinking of getting Business Starter package from Workspace https://workspace.google.co.uk/intl/en_uk/pricing.html

But Im not sure how many custom domains can i add in the Gmail business email?

It says £4.14/user as in per domain or mailbox?

I have multiple mailboxes with custom domain for each mailboxes.

ie. sales@example.com, sales@company.com, sales@business.com.

thanks in advance