Well, let's assume a greatly simplified fleet composition for a moment.

In 2280 Starfleet has:

15

*Constitutions* w/complement of 500

35

*Mirandas* w/complement of 350

40

*Soyuz* w/complement of 300

75

*Saladins* w/complement of 200

75

*Hermes* w/complement of 195

For a total of 240 ships, and 61,375 personnel.

Starfleet decides that the

*Soyuz*-class no longer fits their needs, and feel they need more ships to cover a larger number of sectors from Romulans and Klingons. So they mothball the 40 ships of that class, in favor of building more ships. They have the capacity to build all new ships, with a total of 60 ships needed over the next ten years (so 6 ships per year) and in 2290 they get:

20

*Constitutions* w/complement of 500

40

*Mirandas* w/complement of 350

100

*Saladins* w/complement of 200

100

*Hermes* w/complement of 195

For a total of 260 ships and 63,500 personnel.

Khitomer accords happens, Federation Council tells Starfleet that they can keep their current personnel counts, but that they won't get new shipyards or a budget increase for say the next 20 years. Starfleet's equivalent of the Admiralty Board then decides it'll mothball/decommission everything but

*Miranda*-cruisers and bigger, badder

*Excelsior*-class explorers. Since they don't get an increase in personnel numbers and they still have to deal with 6 ships per year, they start the slow process by building 4 new

*Mirandas* and 2 new

*Excelsiors* per year. So over the next 20 years they do exactly that and accounting for losses and the like they end up in 2310 with:

31

*Excelsiors* w/complement of 750

115

*Mirandas* w/complement of 350

Total Ships: 146, Personnel: 63,500

Now at this point the Federation Council says: "Well hey guys, we now have a lot more space to cover thanks to your exploratory efforts and we have a bigger budget, we can let you double the size of your fleet and shipyard capacity." Given the realities that Starfleet has a very large number of both

*Excelsior* and

*Miranda*-class ships about (and their supply chain/maintenance chain is biased very strongly in favor of those ships), they decide to maintain those as the mainline ships (with substantial modernization of course) for the time being while expanding their shipyard capability and building 8

*Mirandas* and 4

*Excelsiors* per year until 2320.

Accounting for losses we'd see a simplified fleet of something like:

175

*Mirandas*
60

*Excelsiors*
Total: 235 ships, Personnel: 106,250

Now by 2320s, Starfleet now has no more Romulans to deal with, they're richer than ever, but see that the Klingon Empire is starting to make noises again. As a show of force they start building new modern ships based on new technologies while retrofitting older ships with service life extensions/refurbishments. So about 2/3rd of their shipyard capability is taken up with repairing/upgrading their old ships (which can do this in 6 months) vs. the remaining capability used for building new ships. Half of the capability is used to build modernized

*Excelsiors* and

*Mirandas* in alternating years while the other half build brand new

*Ambassador*-class ships which require 2 years to build vs the previous 1 year build-up. From 2320 to 2330 then (and assuming some losses) Starfleet changes to become:

165

*Mirandas*
60

*Excelsiors*
10

*Ambassador*
Total: 235 ships, Personnel: 111,750

You can continue a process of this sort well into the 2360s where you eventually end up with just 6

*Galaxy*-class ships being built in the 2350s, explaining why you see so many

*Mirandas* and

*Excelsiors* around. By building up such a huge store of them at some point in the 2290 - 2310s, Starfleet must've had a vested interest in continuing to develop technology/modules for them to upgrade with.