My overall suggestion is to sit down with all of the players and talk about what the group expectations are for how often people will be able to make it, and how often people can miss due to emergencies and other events. A player that has too many emergencies may have very good reasons to have those emergencies, but they are still disrupting a game by not being there and may not be a good fit for a weekly game. They may be better suited to a game that means less frequently.
However, even when you have everyone dedicated and not prone to emergencies, emergency still arise. My Group has a rule that if we are missing one player we NPC that character for that session, but if two are missing we switch to board games. There are a lot of amazing board games out there these days that are just as fun as D&D. You might miss out on advancing the story for an extra week which can be really painful when you're at a really fun part of the story, can be amazingly fun. if you don't own any board games created in the last 15 years, I suggest you go to the website called BoardGameGeek and look for some suggestions. Lords of Waterdeep, Gloomhaven and Terraforming Mars are all great games for a D&D group to enjoy when a player is out.
