by whatever criteria I use to decide what to count
1,500 is a big number! I don't know how many I've done (I have good records for some years, not for others). Surely more than 1,000 but I'd be really surprised if it was as many as you. Though perhaps I have you on the time between first and last races (56 years)? :-)
Suppose you run two legs in a relay. Count as two races? It would seem so. There was this relay I ran in in Switzerland, 10 legs, I ran all ten (28 km, 1,700 meters climb, 63 controls). Got to be worth more than one race.
Can't recall offhand whether I've done that (I'd have to check the database), but if most other teams were having multiple people do those legs, then yes, I'd count them as separate courses. As opposed to a motala, which would just be one. Your relay would be 10.
My span is only 41 years (starting December 1978), and '80-'82 are blanks. Your span is 15 years longer, but you're 17 years older.
I may be competitive in the span (55 years) but totally non-competitive in the number! (Long gaps of laziness, and lack of time and inclination to travel too far for the sport)
I found eight instances where I ran two legs in a relay, but I think in all cases, they were four-leg relays run by teams of two, alternating legs (some of these were at Ratlum Mountain). I counted each of those as two courses.