In an octahedral complex, the d-orbitals split into t2g and eg levels.
For d3: Configuration is t2g3eg0. Number of unpaired electrons = 3.
For d4 (low spin): Configuration is t2g4eg0. Number of unpaired electrons = 2.
For d5 (high spin): Configuration is t2g3eg2. Number of unpaired electrons = 5.
For d6 (high spin): Configuration is t2g4eg2. Number of unpaired electrons = 4.
For d7 (low spin): Configuration is t2g6eg1. Number of unpaired electrons = 1.
Total number of unpaired electrons = 3+2+5+4+1=15.
Answer: 15