A raiding guild for 40 players is typically composed of no less than 60-70 regular players to deal with precisely this problem. Top guilds will often have enough players to fill three or more raid groups in order to have enough prepared players for the "main raid."
That's a bit of an exaggeration. A lot of guilds can barely fill their raid roster. They manage by strongly pressuring all raiders to show up every day on time, and to give advance notice if they need to be out.
Speaking personally, until very recently my regular raiding guild only had about 26 or so raiders who could show up every night (and another 3 or 4 who could show up for 1 or 2 of the nights). And yet we still managed to clear 8/8H.
