Here are some comparisons so far
XY: 11.61mil vs ORAS: 9.35mil (years end from release)
New Gen Pairs
♥31.38mil: Pokemon Red/Blue/Green
♥23.10mil: Pokemon Gold/Silver
♥17.63mil: Pokemon Diamond/Pearl
♥16.22mil: Pokemon Ruby/Sapphire
♥15.58mil: Pokemon Black/White
♥13.29mil: Pokemon XY (From end of Sept)
Third Installments/Direct Sequels
●14.64mil: Pokemon Yellow
● 8.52mil: Pokemon Black2/White2
● 7.60mil: Pokemon Platinum
● 6.39mil: Pokemon Crstal
● 6.32mil: Pokemon Emerald
Remakes
♡12.72mil: Pokemon Heartgold/Soulsilver
♡12.00mil: Pokemon FireRed/LeafGreen
♡ 9.35mil: Pokemon OmegaRuby/AlphaSapphire