Cool thanks for the insight! The joy (and possible frustration) of Star Trek lore was that not everything was explained thus giving that mythos where you want to fill in the blanks with conjecture. Problem is it seems even those working within Trek tried to do this and some cases didn't have the answers themselves or made matters worse by adding contradictions. So yeah it does seem likely that while only 12 ships "like the Enterprise" exist, there were likely many more ships which looked just like her and we will never really know the answer as to when the design first started and how many were ever truly built. That's quite fine by me though I'm happy to conclude that the original USS Constitution may have had a registry as far back as the 900's and that ships of this design might have been churning out for decades. It would go a long way to explain why by the 24th century it had been phased out while the Miranda kept going as that design was still new in the 2280's.
BTW I'd love to find further reading on this if possible as I'm sure I can't be the first person to come to these conclusions.