In the Jefferies model, the implication is "16th cruiser design" and "18th cruiser design." By that method, there are/have been 98 16xx class, 19 17xx class, and 32 18xx class vessels indicated by that chart. (Not to mention the 18 10xx vessels.
Serial numbers make it even a larger fleet, with 813 ships between NCC-1017 and NCC-1831, although not all of those are necessarily starships.
I'd like to revisit the issue because today a different idea came to my mind, mostly inspired from "The Doomsday-Machine".
In the episode's beginning there's heavy subspace interference:
PALMER: The distress call definitely came from one of the solar systems in this sector.
KIRK: Can you pinpoint it any closer, Lieutenant?
PALMER: Negative. It was so badly garbled all we got was the name Constellation, then we lost it.
The Epsilon 9 subspace chatter from TMP hinted that a proper subspace message will contain the name of the vessel and its NCC registry.
If Lt. Palmer would not have gotten the name in the distress call but just a fragment of the NCC registry (e.g. "...17") would that have meant they remained clueless (too many NCC registries ending with a "17") or would that have helped them to immediately identify the Constellation
by the "17" ending?
Apparently there'd only be a pool of 99 numbers available to the front-line Starfleet vessels (those most likely to experience subspace disruption and possibly limited to starships and destroyers).
The last two numbers follows no chronological system but will be assigned ("Naval Contact Code"?) based on availability. If a vessel is scrapped or destroyed or considered permanently lost it's removed from the active inventory and its last two NCC digits can be assigned to a new vessel.
Interestingly that works well with three known Star Trek starships.
- USS Excalibur (NCC-1664) was destroyed by the M-5 in "The Ultimate Computer", the last two digits passed on to the
- USS Defiant (NCC-1764) whose crew was found dead and the ship was considered permanenty lost in interphase in "The Tholian Web", the last two digits passed on to the
- USS Reliant (NCC-1864) that was destroyed in the explosion of the Genesis Device in "The Wrath of Khan".
(Darn, I would never serve on a starship with a registry ending of "64"
Admittedly, there'd be something wrong with the USS Intrepid
(NCC-1631 according to TOS-R) as it existed at the same time Commodore Stone's starship status chart in "Court-Martial" (now in HD) shows the number to be actually NCC-1831
But since Greg Jein's "1631" was a misinterpretation of the actual "1831" (indiscernible to read properly when he wrote his "John Doe Starship" article) I'd say either TOS-R is wrong or the starship status chart needs a fix.