There are two catastrophic scenarios for Congress: The death of a large number of members and the incapacitation of a large number of members. Each comes with massively different outcomes in terms of its impact on legislative business and massively different outcomes for democracy.
The outright death of members of Congress puts in motion a relatively straightforward process, albeit a potentially slow one. The Senate, with its tradition of appointment by state legislature, largely has procedures to allow itself to reconstitute quickly through gubernatorial appointments, followed by special elections. The House, which prides itself on direct election by the people, has long fought any effort to allow for emergency appointments to the body, so any vacancies would be filled only by special elections, a process that would take months.
As representatives and senators die and vacancies arise, Congress can keep legally functioning no matter how small its remaining elected membership becomes—even though a notably smaller Congress might find its public legitimacy in question. Norm Ornstein—the scholar who has most consistently raised alarms about congressional continuity—pointed out after 9/11 that Congress has since the Civil War interpreted the quorum necessary for it to function as consisting of the majority of members “duly chosen, sworn, and living,” a definition settled on as a workaround to avoid the unpleasantness of the many senators and representatives who departed as the Confederacy seceded.