One card per person, not per membership. Each card carries that person's name.
On a household membership, each person on the household can have their own card. On an organizational membership, each person currently in a seat can have one.
The Membership cards panel on the member record lists everyone the membership covers. Each person shows No card yet, Issued, On a phone or Voided, with a Send card button that emails that person the link to add their own card. The button is greyed out for anyone with no email address on file.
When a person comes off a household, or leaves a seat, their card is voided. The rest of the membership, and everyone else's cards, are untouched. A new person in the seat gets a card of their own.
A person's card is one card whichever wallet it is in. On an iPhone the panel shows how many devices it is installed on.