It is estimated that apaths make up at least 65% of people in a given population. It has been tested using bystander studies, obedience studies and the results of the empathy quotient testing.
The empathy bell curve is shown below. 20% are thought to have a high empathy default position, even less are exceptional altruists. Between 1-4% is the usual estimate given for prevalence of antisocial personality disorders with a 10% of the population in the grey area (everyday sadists).
Source: The Empathy Trap