I avoid the words ‘normal’ and ‘abnormal’ as there is no absolute value below which pregnancy cannot occur, but fertility specialists like to see a count of greater than 20 million, or, better still, greater than 40 million per ml.
After a vasectomy reversal, however, things are different because the man will have had children in the past (before his vasectomy) and hence the quality of the sperm will be OK. 12 million per ml is a good count after vasectomy reversal.
The one common complication that vasectomy can lead to is the formation of anti-sperm antibodies. Once these are bound to the sperm, they can prevent fertilisation. The only reasonably effective treatment for this is Intracytoplasmic Sperm Injection (ICSI), which is performed as part of an IVF cycle. Any IVF unit can test sperm for the presence of anti-sperm antibodies.