I have done a lot of study on game rarity, and this is what I can tell you:
First and foremost, it is absolutely impossible to get a finite number. It simply isn't feasible. Some people have burned them, parted them out, don't even know they have one in the attic, or there may be a stash of 50 somewhere in a warehouse.
As far as Rocky goes...1504 were made, 53 are registered on ipdb, and 42 on pinside. This does not account, however, for the items mentioned above. It also doesn't take into account multiples owned by a single person, parts owned that could fully make one, or cross-posted pins owned by one collector.
The older the pin, the fewer survivors. That goes without saying, but it is important to note that Rocky came out in 1982, which was in the middle of a huge arcade boom. Many pins we're routed to death, parted, and literally thrown away afterwards. Rocky does have the license tie-in that likely led it into more collector hands, however (especially for the time, when there were far fewer home collectors) so there may be some really nice copies unaccounted for. Due to the popularity of the movies, it is highly unlikely there is a large stash because they would either go to collectors who loved the movies or operators who would route them instead of stash...the lower production numbers also mean that all were likely sold, so there weren't extras laying around.
Rarity and the beloved licensed tie-in mean that a fair amount of them have been restored, refurbished, etc. Unlike a pin with a run of 10k, there likely aren't nearly as many projects laying around, however...it also means a larger percentage were parted.
Pinballs from this era dwindle by approximately 30-60% every decade. That's not to say that they have necessarily been destroyed (though some certainly have), but they have become parted/lost/destroyed, etc. If we assume a 50% depreciation per decade due to the era and its' arcade usage, that puts us at 94 after 4 decades. I would guesstimate a few more are in long time collections, attics, or could be made via extra parts. There are obviously some that are languishing as projects or are just unaccounted for also, but this number is likely not very high for the reasons described above.
To summarize, given the information at hand...100-200 working examples is likely a great guesstimation, with another 100-200 in project condition or being possible from parts.