Well, there are a couple of things we do know that might add to the discussion.

Heuer made a lot of watches in 1970, it was their record production year pre-TAG in fact.

Production decreased for 1971, but it was still their second highest year.

Production continued to decrease, though less steeply, throughout the remainder of the 70s.

Now, given that 1970/71 are probably the two years where most of the Sifferts would have been manufactured, the answer could well be "Not as few as people might be inclined to think". Or "More than expected" for a less subtle take on the same answer.

I'd need to give more thought to a more precise answer. But my immediate instinctual (and researched) feel would be in the thousands, not in the hundreds, which is where I think some people might want to go. Not many thousands, certainly, but more than 1000.