Only one Airtable account with “creator” or “owner” permission is required to set up an extension (e.g. Form or Portal) then it can be shared with an unlimited amount of people. You would not have to pay extra for those people nor need to have Airtable accounts.
Some plans have limits on record creation and editing. However, accessing records without making updates (read-only visits) has no limits at the moment. Users can browse different records with unlimited page views.