According to the APC numbers, the number of active pilots in the various industry segments is, roughly, as follows:

Legacy(+FedEx/UPS/Southwest): 52000
Other passenger carriers (larger a/c than regionals): 7000
Regional: 22000
121 Charter/Cargo (except FedEx/UPS): 5000
Fractional: 4000

Thats 90,000 total pilots. Doesn't include pilots employed flying non-fractional, non-121 type stuff.

---- Disclaimer: Everything after this line is wild assumptions on my part, just playing with the numbers ----

Now, time to make some big assumptions. Lets say that the percentage of pilots in each market segment with applications to a legacy are as follows:

Other legacies: 10%
Other large pax carriers: 50%
Regionals: 75%
121 Cargo/Charter: 75%
Fractionals: 25%

Thats over 27000 pilots actively pursuing a job at a legacy carrier. Lets say that each airline only gets an application from 1 out of every 4 of the pilots pursuing a legacy. Thats about 7000 applications.

Now lets say that only 50% of the regional pilots applying are qualified. That takes 8000 pilots actively pursuing a legacy out of the loop. Lets assume 90% of the other pilots are qualified... not too many low timers or those without ATPs/Types in the other market segments. That takes another 1000 out of the loop. That leaves us with 18000 qualified pilots pursuing a legacy.

The 1 out of every 4 part puts it at about 4500 qualified applicants at each airline.