The price listed is not per Airtable user. Only one miniExtensions subscription license is needed and it can be used by all your users in Airtable.
Regardless of how many collaborators you have in Airtable, you only need to pay for one subscription then you can have unlimited users access the extensions you build with no extra fees.
It's the same fixed rate, whether you have 1 user, 100 users, etc.
Only one account is needed to build the extensions. Your end users who need to access your links (e.g. forms/portals) do not need to have a miniExtensions account.