1974 D&D (OD&D)
1977 Basic Set (Holmes Basic)
1978 Advanced
1981 B/X (Moldvay Basic)
1983 BECMI (Mentzer Basic)
1989 Advanced 2e
1991 New Easy to Master
1991 Rules Cyclopedia
1994 Classic
1995 Advanced 2e Revised
2000 3e
2003 3.5e
2008 4e
2014 5e
2024 5.5e
Congratulations to D&D 5.5e, the 15th edition of D&D!
(I'm not familiar with New Easy to Master and Classic so if those are just variations of Cyclopedia then it's 13. but also individual "rules editions" had different revised printings aside from 1995 2e- my copy of Holmes is the 3rd printing replacing Hobbits with Halflings- so if you include those who the hell knows)