To be sure, we don't have direct evidence of any ships being built during the war. The only "new" ship to emerge would have been the Sao Paulo
, but she might also have been in existence and service/mothballs since before the war, even though the implication is that she's fresh off the docks.
It might be that it takes several years to build any reasonably sized combat starship. Which would probably mean that Starfleet would
gain significantly in ships - but at the very end of the war, rather than during it. Whether that gain would balance the losses is unknown.