7,000 active ships might be good enough for Starfleet of TOS to rule sovereign over the UFP territory of TOS (and they really love to rule sovereign, putative central government or not!). But an expanding UFP would soon have 7,000,000 active ships for its Starfleet, unless active measures were taken to prevent the building of the ships and the drafting of the crews. And then 7,000,000,000 ships. What's gonna stop them? Or even slow them down?
ST: Discovery stated that at it's peak @ ~ 3069, the UFP had 350 members.
Even if StarFleet had 7 Billion Ships & , and they were in the 32nd Century and HYPOTHETICALLY, the burn never happenned and StarFleet is at full strength, there is no Dilithium Shortage and they can make Synthetic Dilithium from scratch.
You still have 4 Quadrants to watch over, and not every ship will be active at any given time.
That's 1,750,000,000 assigned to each Quadrant.
Now IRL, the US Navy has a rotating schedule where various ships are in different status:
I've come up with a simple 24 month schedule O-FRP (Optimized Fleet Response Plan)
Maintenance and/or Repairs: 01-08 months (In Dry Dock or at a StarBase doing routine maintenance and getting checked out, or getting repaired because the ship is seriously injured.)
Ship ShakeDown / Training: 04-08 months (Validate that every system on the ship is functioning as expected, then get the Crew Up & Running, synced and working like a well oiled machine)
Travel time & Deployment @: 08-19 months (Travel to your mission objective & spend the majority of time @ that location, then travel back)
Obviously that means various ships can be out and about while others are at Dry Dock sitting for a while.
So assuming a simple math of 1/3 of your fleet in one of those status at any given time for arguments sake.
You'll only have about 1/3 of your fleet: 583,333,334 out and about in your quadrant exploring, doing normal UFP deployment stuff.
Another 1/3 in Dry Dock, and another 1/3 in training.
Or some mix of shifting a few more in each bucket.
Don't forget, not all ships are out "Exploring" the final frontier.
Some are on:
- logistical duties
- 2nd Contact
- Border Patrol & Tactical Security in a region
- Long Term Scientific Studies
- Long Term Engineering Projects
- Etc
Not everybody gets the glorious Explorer jobs where they spelunk through the Final Frontier.
And don't forget how vast space is in 3 Dimensions.