There were a lot of them in the Dominion War, and judging from registry numbers, it looks like the Excelsior and Miranda were in full scale production. There was an Excelsior or two in the fleet meeting the Borg sphere at the end of Voyager, too.
Though that brings me to something I've always wondered: There were a huge number of TMP-era ships in Starfleet in the TNG/DS9 timeframe, which I can believe since tech manuals say ships are built to last upwards of a century or two with refits and upgrades. But there's a big hole in the ships seen on screen. TMP-era ships are the bulk of the fleet, and TNG-era ones (Galaxy, Nebula, Akira, etc) are the backbone, but the things in between, like the Ambassador, Cheyenne, Constellation, and so forth, are almost entirely absent. Most of the canon ship classes from that lost era of ship design don't even exist except as Wolf 359 wreckage. There should still have been a substantial number of them in service in that timeframe, if not also in production as the older ships were.
Maybe it's a new production Excelsior. Maybe it's in drydock for repairs. Inconclusive.
Actually... look closely...
You can see that part of the "left?" side of the saucer is missing, but not like the Enterprise E when it was undergoing repairs in Nemesis, more Complete... This would indeed indicate direct construction rather than repairs...
Also... Seriously... Some of you can't see the excelsior? w00t?
This is from 2371, a new Excelsior, or a refit of an older one is going on.
I'm on the whole "this ship should be tier 5", vs the "this ship is over 100 years old!" argument. Just saw this and thought it was cool.
I'd just chalk it up to using existing assets for a background shot. Star Trek is notorious for reusing assets (reusing the old Excelsior model for Star Trek Generations, kitbashing, etc.) to keep the budget down.