I feel like there are several general observations to bear in mind.
1) The nature of registries
. Be they contact codes or construction contracts, one must rationalize that they are at least somewhat unique to the vessel to which they are assigned. The general pattern we've observed has been numeric and incremental over the years. To my mind, this does not necessitate that registries have always followed the same pattern. For example, a 23rd century Starfleet assigning registries based on tonnage/class (compare the somewhat small Antares
with an NCC-501 registry to the NCC-1701 Enterprise
, for example) but the 24th century Starfleet assigning them soley based on production order, seems possible. I also think it's likely the 22nd century Starfleet had a different paradigm, given the prefix (NX, etc) seems to have been unique to each class. Consider also that Starfleet may want to obfuscate exactly how many starships she does have in service at a given time. Using blocks of registries and then skipping ahead might actually prove beneficial.
2) Sequential registries vs. continuous registries.
It appears to me that the general intent has been that lower numbers predate higher numbers, with a few exceptions. Meaning, NCC-1700 came before NCC-2000. However, I see nothing to prove that every number between NCC-1701 through NCC-1999 actually exists, nor anything to prove that ships higher than the NCC-2000 range might've been commissioned before the Excelsior. On the other hand, we see nothing to disprove it either. This point becomes particularly contentious in the 24th century when registry numbers become so high. To me, this indicates that registry numbers are (at least by this time) single use and retired when a ship is lost. Furthermore, Starfleet may reserve certain registry blocks for specific purposes that may never come to fruition. It seems rather convenient that the fancy Great Experiment was NX/NCC-2000.
I see nothing to indicate that large numbers of ships stay in service for a long time. Clearly there are certain older vessels that have remained in some form of service into the 24th century, or at the very least were put into mothballs and later reactivated. I would compare the number of B-52s that the U.S. built during the Cold War. So, just because you have NCC-75000 in service in 2375 (I pulled that out of the air) doesn't mean there are a total of 75,000 starships in service.
Filmed evidence would seem to corroborate the notion that there is maybe one starship per sector of the Federation (given how often our heroes find themselves "the only ship in the quadrant"), and the gathering of these in one place is very rare. Given that the 24th century Federation is 8000ish light years across, and sectors are a few light years across, 2,500 commissioned starships seems very plausible to me, with perhaps a few hundred in reserves and/or in inactive 'home guard' duty. The registry of those 2,500 ships might range from NCC-10000 to NCC-77000, but not include 67,000 vessels.
As much as I'd like to allow for the idea that registries somehow correspond to identification beacons, I don't see anything that actually supports this, nor is there really a real-life equivalent. I think simple serial numbers, be they based on class and model or simply incremental and somewhat arbitrary, is most likely.