Q3 had 19 DM maps, 6 tourney maps and 4 ctf maps. Three times as many maps, but it was designed as a multiplayer only game. Just like Q3 I think we can expect those numbers to be dwarfed by community generated content.
Not to mention that most of those maps were quite bad. Q3Test had only one map, Q2 was released with no multiplayer maps at all (it took another three months to get those).
Nine (or any other positive number of) multiplayer maps in the new Doom is all right.
if they make dm6 in true doom4 fashion that would be out of this world... so much out of this world it would probably be in hell, which means dm6 should look like hell.