Typically, I will call the game off if I have two or more players missing (assuming 4-5 players). First off, I don't like running "side adventures" and I don't like progressing campaigns with only two or fewer people. Second, if we're all friends with each other, having the other players denied a game helps put pressure on them, and the no-shows to show up, or support getting new people. I've found its pretty easy when one player misses, even if they're missing regularly, for the other players to fall into a rut of "this is okay", even when it is highly irritating to me as the DM and the attitude makes it worse, because what they're missing is that now if one more of them misses the whole game gets called....anyway.
Constant flakes get moved to "cameo" status. They're free to show up wherever the players are, help out and get credit for that day, but they get none of the long-running benefits of being part of the party. Their characters are not allowed to be run any anyone (as a person who misses infrequently would be allowed) while that person is out, they are essentially no longer part of the party. They can miss out on anything from loot, XP (when I use it), levels (when they miss milestones) and in-game reputation earned with the factions of the game.
Now, there's a special circle of hell for last-minute flakes. It's one thing if Bob says on Tuesday he can't make Friday's game. But if Bob days nothing until an hour before the game, and does it frequently, Bob gets not so kindly asked to either get his stuff together, put it in a backpack and take it somewhere, or to stop attending. The same applies to people who do the reverse, saying they won't come until the last minute and then saying they're gonna show. I try to make a decision on if we're having a game at least 24 hours in advance, so if you can't give me the same courtesy, you clearly lack courtesy and are not someone I wish to play with.
I may struggle to find players sometimes, but when I find ones, they're amazing and I'm okay with that. There's this one 15-year-old kid I game with, I'll take him over 70% of the adults I've ever gamed with, even if this kid likes to play evil characters. He's polite, he's thankful he's always timely and always knows whats going on with his life over the next couple weeks and if he can make a game. He gets the game (both the math and the social aspect) and man, just thinking about my players makes me sad that I haven't played in a couple months.