I place a big store in keeping a regular schedule, so that my players know that on this weekend and that weekend, at this time, ever month, we game. People can plan their own schedules accordingly.
If only one or two people show up to play, then I still run the session. It’s pretty easy to adjust the encounters downward, and I don’t believe in telling the people that showed up that they can’t game because other people were slackers (or, you know, had things they actually had to do instead).
Sometimes (one of my groups has some players that can no longer be relied upon to consistently show up), I will also plan in advance for those epic boss fights at separate tiers. So a full group might get that fight with the dragon, plus a set of wyrmlings, plus pools of magma. If it’s not a full table, then I ignore the wyrmlings, and maybe the pools of magma. Sometimes I even completely change out the monster. It’s more prep work, but it makes it easy for me to run an adventure and not worry about scrambling at the last minute.