I've noticed that, in order to function correctly in a mission, a ship needs a minimum of 1 character in each department, plus CO and XO. That makes up 8 people per ship. To have five ships in the Fleet, not counting the Delphia A, that would make up 40 personnel. I would normally say a maximum of 3 players/department/ship otherwise logistics of posting get complicated.
Going by the memberlist, we currently have 51 characters active, 8 of which are command level plus 5 not assigned to ships, including the admirals. Take off another 7 for the secondary characters I counted gives us 44 characters including the command staff. Take out the admirals, gives us 39
Basically, one more player would theoretically be enough to make the minimum, but that's not taking into account the fact that we don't all have the same activity levels, as well as the fact that it's a lot more fun with more players.