Rationale: Let's assume a home-run rate of about 3% which is my rough estimate of what it is for YC startups. That means that funding 75 startups yields a 90% chance of at least one home run.
Now one might ask how long this will take. If we assume that 50% of the companies can be marked as failures quickly, he will only need to invest substantial time in ~37 companies. If he can focus principally on 3 at a time for a period of 6 months, it will take approximately 6 years to achieve a 90% home run probability. Of course, he'll almost certainly have had some modest successes before then.
Again this is all predicated on hmexx being good at selecting and advising startups. For most people, the odds are much worse.