One of the hallmarks of a West Marches campaign is a large pool of players, more than can fit at a table at one time. In an optimal world, the players get together as groups and schedule the games and ask the DM to be there. This works great in high school or college (particularly college), but after that it gets a bit more dicey.
What I am finding is that the game day is set based upon when the DM can be available and then players who can show up to play. A certain number of players can usually make every game with an equal number being more sporadic. The sporadic players will stick with the game until their character start falling behind in experience and then they drop out entirely.
This seems to lead towards a more standard gaming group, but I think it has greater flexibility in the way of bringing in new players. There is motivation to keep making games to avoid falling behind and with a more gygaxian naturalism, there is always the chance that high level PCs may die and restart, keeping overall levels relatively close (in theory anyways).
[Note: by "gygaxian naturalism" I mean the world is a somewhat naturalistic environment and what "should" be there actually is, regardless of level-appropriateness. So if your 5th level party wanders into a pride of dire lions, they better get out of there quick before they become kitty kibble. The opposite of this is a series of level appropriate encounters that gradually rise in difficulty to match the PCs increasing capabilities, as you see advocated in 3.x and 4E D&D.]
Now, one of the downsides to this is that sometimes you don't have a lot of players if real life conspires that way. In my Southern Reaches game, I have 10 players in my pool of players, but only regularly see six of them at the moment. In the Verden game, we started with six players and last session only had three. The question becomes, "Do I need to recruit more players or let it ride as-is?" The answer seems to be, "It depends."
For the Southern Reaches game, I don't think I need to recruit more players, partly because I still have six, but also because the reasons keeping the other players away are temporary and will be ending in the next month or so. For the Verden game, the DM is asking us if we know more players and I think he is right to do so. Three is a difficult number of characters to adventure with, especially if we do not have access to hirelings to supplement our numbers. The missing three players will likely be back, but it is much easier to run with a few more players and would improve our confidence that the game will regularly make (especially as I tend to be unavoidably late).