We really don't know the thresholds of severity of head injury and numbers of incidents resulting in CTE nor the timing. A little less than 20% of those diagnosed with a closed head injury get CTE. We think the more often there is injury and the more severe it is leads to a higher incidence of CTE but we don't really have a handle on it yet. One problem is that it's difficult to diagnose it until post-mortem examination is done and symptoms and signs of CTE are seen in other disorders as well.
Did you know there are only 5000 Snow Leopards in the wild now and they are confined to Central Asia? However, the effective global population (those likely to reproduce) is less than half that number.