As an amateur pilot I'm aware that I can lose my medical at some point and have to give up flying. For a professional, losing a medical can end their career. I know several people who wouldn't even get a PPL with their medical history. Are their any figures on the number of CPLs/ATPLs that lose their livelihood for this reason?
This information doesn't easily exist, I'm sure insurance companies have some figures but they don't seem to be public.
I'm just going to expand on Pondlife's answer with more data. In Australia the regulator CASA publishes all sorts of statistics, including the number of medical certificates issued and refused (page 164).
Australian professional pilots need a Class 1 medical certificate. In the 2017-18 financial year CASA issued 18,155 certificates and only refused 26, or 0.14%. Over the past 5 financial years, 0.22% of applications were refused, and the trend is decreasing.
Some caveats on this data:
- Not all of this 0.22% were already professional pilots, in fact I would guess that a significant amount were prospective pilots seeing if they would pass before investing so much money in training.
- These numbers only include those who actually applied for certification. If a pilot suffered a major health diagnosis they probably wouldn't bother to even apply.
- Class 1 certificates are not just for airline pilots. GA pilots, instructors and so on are also included in this percentage.
- CASA has a reputation of being a bit more restrictive than the US, but are more allowing on some particular conditions.
So the answer to your question is more than 0, but not enough for a person of ordinary health to be worried about, in my opinion.
The problem with answering your question is that it isn't all clear what "losing a medical" actually means. If a pilot is denied a medical, the FAA will obviously know and you could probably get that data from them somehow, even if it takes an FOIA request. But what if you just decide not to renew because you know you'd fail? For all the FAA knows, you might have retired, changed career or even died.
If we assume you do mean "denied" then the only specific number I could (quickly) find for denials was from 1978:
The annual denial rate based on airman applicants is 7.9 per 1,000 airmen. By class of certificate applied for, the annual denial rate per 1,000 applicants is 4.5 for the first class, 6.0 for second class, and 10.2 for third class.
You could apply those rates to the number of airmen certified each year to get an estimate of the absolute numbers, but obviously using a 40 year-old report may not be accurate today. And determining how medicals 'map' to pilots may not be easy.
If you want a better answer, you'll have to define exactly what you're asking. To get an accurate number (whatever that means for your purposes) you might have to look for some specialized actuarial or medical research on this.