I will run the scheduled game if less than half the players are missing.
However, I also keep some side games prepared so that there is a game to play for the players that show up. So for Friday nights the schedule is alternating sessions of Shadowrun and a home-brew D&D campaign, but if it is only N. and S. available to play we've got a Dark Sun campaign for just those two, or if it is just R. and S. we have a side story of the home-brew campaign with just those two, or if N., R., and S. are there we play a Rappan Athuk campaign, and so on.
But I definitely don't let anyone show up for game night at my place and not be entertained - and I only cancel game night if I just physically can't be entertaining that night.