Here’s my theory for the Enterprise-A:
During STIII, Morrow mentions that the Enterprise is to be decommissioned. What he doesn’t say is that it’s because a new Enterprise-A is being planned, an Excelsior class ship. But because of the events of STIV, Kirk was awarded another Constitution class ship like his old one, and it was named the Enterprise-A in his honor (and the fact that the original Excelsior class Ent-A wasn’t finished being built yet), so instead the ship builders made the Excelsior class the Enterprise-B, and Kirk was just given command of the A temporarily until the B was commissioned. That’s why the A was decommissioned so soon after it’s construction: it was time for the B to be christened. The A was probably recommissioned after that with a new name and registry number.
During STIII, Morrow mentions that the Enterprise is to be decommissioned. What he doesn’t say is that it’s because a new Enterprise-A is being planned, an Excelsior class ship. But because of the events of STIV, Kirk was awarded another Constitution class ship like his old one, and it was named the Enterprise-A in his honor (and the fact that the original Excelsior class Ent-A wasn’t finished being built yet), so instead the ship builders made the Excelsior class the Enterprise-B, and Kirk was just given command of the A temporarily until the B was commissioned. That’s why the A was decommissioned so soon after it’s construction: it was time for the B to be christened. The A was probably recommissioned after that with a new name and registry number.
All ships in the starship class are called Starships regardless of the design subclass or hull class of the ship (i.e. Constitution Class, Miranda Class, etc.). Starships are heavy cruiser type spaceships specially equipped for deep space duty (longterm patrol, exploration, and scientific investigation). "There are only 12 like her in the fleet." I figure Kirk is counting all "starship class" ships and not just "Constitution class" subclass ships.