5
Explanation:
The maximum numbers of d orbitals are 5. These are named as dz², dxy, dyz, dzx, dx² - y².
The number of maximum electrons accommodate in d orbitals are,
l = 2
2 ( 2(2) + 1)
2( 4 + 1)
10
so maximum electrons in d subshell are 10.
The 1st and second energy level do not have d orbitals. Third energy level in addition to s and p orbitals contain d orbitals.
Energy level      sublevel        N of orbitals          No of electrons
  1                s               1                   2  Â
 2                s               1                   2
                  p               3                  6 (total=8)
 3                 s               1                   2
                  p              3                   6
                  d               5                 10 (total=18)