Starfleet clearly does have a massive mothball fleet in peacetime. It is mentioned in ST6 (albeit briefly)
Actually, there is talk about the possibility
of mothballing Starfleet, and everybody seems to think it's a piss-poor idea! Clearly it's not standard practice at the time...
In "Unification", we see some Starfleet vessels (mostly Mirandas
-related study models) in "mothballs", although that expression is not used. We don't learn whether the ships are in any condition to be reactivated, or merely hulks too badly worn out to ever sail again. At least one unseen vessel is stolen in the episode, but apparently piecemeal, with the help of another vessel.
and we see so many old Excelsiors and Mirandas in DS9 we can safely assume they were not all active before the start of the war.
Yet we did see a lot of them active before the start of the war. I'd rather believe in Starfleet keeping all its Excelsiors
fully up and running in peacetime, so that the newer Akiras
(which for some reason don't appear in the peacetime of TNG) can remain in pristine condition for as long as possible...