"The couple will have died."
Translation:La pareja habrá muerto.
"Pareja" is a singular word, that's why you use LA pareja (In one "pareja" there are 2 people, but the word is singular). If you are talking about more than one "pareja", you use LAS parejaS, so in that case yes, you are talking in plural.
Examples: La pareja compró pan (the couple bought bread). Las parejas compraron pan (the couples bought bread).
You can compare this with the concept "group" in english. A group coud be a lot of people, but you talk in singular about them: "IT IS a large group of people" (the people could be a lot, but the group is just one).
This reference from the website Espanol avancado, says differently, and I can say with certainty that in Mexico the reflexive is used in this situation, though perhaps less often in formal Spanish. https://www.espanolavanzado.com/gramatica-avanzada/28-uso-de-palabras/439-morir-vs-morirse
"morirse is preferred when it is a natural death, especially when referring to the death of friends or relatives, where we are focusing on the subject of the verb, describing what the subject 'did', so to speak (far more common in spoken/informal Spanish).
With the pronominal form, the verb morir tends to be foregrounded in the context and, therefore, is rarely used in subordinate/adjective clauses.
-Juan se murió de cáncer hace poco — Juan recently died of cancer
→Su hijo Juan, que (se)murió de cáncer hace poco, vivía en Francia — Her son Juan, who recently died of cancer, lived in France
-Mi abuelo se murió el año pasado — My grandad died last year"