Sales History

In Japan:

(From Famitsu #622, sales up to March 31, 2000 according to CESA, the ranking of the Dragon Quest series was compared to all games in Japan)

Dragon Quest 1 (NES) 1.5 million copies (60th)
Dragon Quest 2 (NES) 2.4 million copies (26th)
Dragon Quest 3 (NES) 3.8 million copies (8th)
Dragon Quest 4 (NES) 3.1 million copies (16th)
Dragon Quest 5 (SNES) 2.8 million copies (19th)
Dragon Quest 1&2 (SNES) 1.2 million copies (91st)
Dragon Quest 6 (SNES) 3.2 million copies (14th)
Dragon Quest 3 (SNES) 1.4 million copies (73rd)
Dragon Quest 7 (Playstation) 4 million copies (new information)

In the US:

(From Famitsu #621)

Dragon Warrior 1 (NES) 500 thousand copies
Dragon Warrior 2 (NES) 150 thousand copies
Dragon Warrior 3 (NES) 95 thousand copies
Dragon Warrior 4 (NES) 80 thousand copies

--Thanks to Charles Hicks for the info