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 I (NES) 1.5 million copies (60th)
Dragon Quest II (NES) 2.4 million copies (26th)
Dragon Quest III (NES) 3.8 million copies (8th)
Dragon Quest IV (NES) 3.1 million copies (16th)
Dragon Quest V (SNES) 2.8 million copies (19th)
Dragon Quest I & II (SNES) 1.2 million copies (91st)
Dragon Quest VI (SNES) 3.2 million copies (14th)
Dragon Quest III (SNES) 1.4 million copies (73rd)
Dragon Quest VII (PlayStation) 4 million copies (new information)
Dragon Quest VIII: 3.7 million in Japan, 1.2 million overseas
Dragon Quest IX: 4.26 million in Japan, overseas sales unlisted
Dragon Quest X: Can't find any numbers due to being spread over so many platforms
Dragon Quest XI: 3ds version sold 1.744.603, ps4 version reached 1.336.180, both numbers pulled from mediacreate's "top selling 2017" list

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