Any game that needs bigger groups of players than they have runs the risk of fizzling out or scaling down in order to keep moving forward. Game Masters will still have to adjust a story to keep from obliterating the players that show up because the confounded mage keeps skipping sessions to go boogie boarding, but there is no reason to suffer long term for a group that just consistently lacks players. One solution is to have the GM run some especially well-skilled Non-Player Characters to prevent the players from getting thoroughly mauled. But that is a lot of work to dump on the GM. A more even-handed solution is to allow each player that is 1) reliable and 2) hardworking enough to run a secondary Player Character in conjunction with their primary/original character.
There are some crucial elements to consider when letting a player play two characters at the same time. First, give some thought to letting the characters share a family tie in game. Having two brothers, one a cleric and one a rogue, that get along and share a sibling bond makes it terribly easy to allow them to share loot and combine notes. This makes it easy for players and the Game Master. Second, make sure the player is disciplined enough to even run one character competently before allowing them to tackle a second. Just because three of the players have two characters each doesn’t mean the fourth player can pull it off. We all know they can’t. The games they bother showing up for they miss half of the action because they are snapchatting or trying to keep their bong lit. There is simply no way they can run both a paladin and a necromancer. Third, and this is important, players must be the ones responsible for keeping track of their stats, money, and all of that other stuff. This means the player has to be trustworthy.
As difficult as it is to imagine I have encountered many people who did their best to cheat at Dungeons & Dragons. They try and hide their dice rolls, fudge the amount of money they have, and even attempt to sneak a spell or two in that they aren’t entitled to. It always amuses me. You are really going to cheat at a game that doesn’t have actual winners? Those folks can’t be relied on to run a second character when they can’t be trusted to honestly run the one they already have. In an ideal world these people will get bored and leave the game. Or move to another part of the state. Or get themselves stationed overseas. These are all winning ideas. But they don’t help a game short on characters. For that both players and Game Masters have to get creative.