Your local aviation authority
Most CAA's around the world publish facts on how many CPL's/ATPL's they issue in a year and usually have trend graphs to show how many issued in previous years. Most will also have statistics on how many active CPL/ATPL licence's are out there and how many have lapsed.
They will also have stats on how many instructor ratings are active and how many instrument ratings get renewed every year. If you know how many people are roughly in the airlines then you can get a rough picture of how many people have acquired a licence in the last 20 years versus how many IR's get renewed and how many instructors are current. This will probably work out to about 30% of people issued with a licence in the last 20 years are actually working in the industry still.
These numbers can be skewed by the amount of foreign nationals getting UK licences and never working in the UK and also the UK pilots who fly overseas and don't keep the UK licence current.