I can go some days without ever seeing one Mustang. Others, it seems I can't walk five feet without tripping over one.
I've witnessed three other M1s in my area, two base models (yellow, orange) and one HP (FJG). But I've only seen each of these once.
S550s of various other flavors are most often seen. S197s, not so much. GT500s, occasionally. GT350s, I've witnessed two in all the years I've been here.
For the Dark Horse, at least three variables impacting sales and sightings come to mind: 1) price, 2) peoples' financial situation, how they feel about the economy at the time and 3) popularity and enthusiasm with respect to a new mark and first year production.
If I attempted to speculate at all about production numbers, I'm confident I'd be wrong.