Discussion on AIPMT

since,one sperm is enough in order to undergo fertilization but why it is considered that less than 20 million sperms is low sperm count to...????

The problem is that the sperm in infertile men with a low sperm count are often not functionally competent - they cannot fertilise the egg. The fact that only 20% of the sperm are motile means that 80% are immotile - and if so many sperm cannot even swim, one worries about the functional ability of the remaining sperm. After all, if 80% of the television sets produced in a factory are defective, no one is going to buy one of the remaining 20% - even if they seem to look normal.

reference: google