Okay... My top 5:
Sims = 17 (Sims 1+all EPs, Sims 2+7 EPs+1 SP)
Zelda = 10 (ALttP, OoT, MM, OoS/OoA, WW, MC, ALttP/FS GBA, TP and PH)
Final Fantasy = 10 (III, IV, VII, VIII, IX, X, X-2, XI, XII, Crisis Core)
Mario main series = 7 (SMW, SMA-S, SM64, SMS, S'MW: SMA2, NSMB, SMG)
Tie: Star Ocean = 3 (FD, SE, TtEoT) Pokemon = 3 (Blue, FireRed, Diamond)
<3 Nintendo Series.