Multiple characters have always been a feature in my games, depending on player interest (maybe a player wants to try out different character concepts) and number of players (to cover holes in the party if only three or less players are participating). I usually play GMPCs as well, which also covers missing roles in the party, though I'd rather not play a whole team of characters myself on top of GMing--maybe one or two, then have at least one player play two characters to cover other roles.

I also try to keep players from metagaming when playing multiple characters and don't give preferential treatment to my GMPCs--treating them mostly like regular PCs, having them act based only on their personalities and what they know (no metagaming with GM knowledge) and giving regular PCs dibs on treasure (my characters take only what the PCs don't want).

I take inspiration from Dark Sun's concept of character trees, which help provide backup in gritty campaigns with frequent deaths or limited healing, and also provide alternatives depending on what types of skill sets are necessary for the adventure. I've been toying with the idea of expanding on this concept to develop what I call "Player Profiles", which is a type of "character sheet" but for players, which instead of keeping track of abilities or inventories they maintain a Roster of PCs the player has in the campaign or game group. Any time that the player plays a character from their roster they earn XP equal to what their character earned, which can be spent on any character in their PC Roster. But no more than half (round up) of the Player Profile's XP can be spent on any single character.

This serves a similar function to Character Trees, but also expand to help manage multiple characters across multiple worlds or settings in game groups that like to play in different game worlds between sessions.