As for the number of Ambassador class ships, I think that ships of the same class are generally built in batches of about 10 to 20, maybe smaller batches for Galaxy clas ships. They don't built too few ships at a time, for there are considerable cost savings if it is done in parallel. They can't build too many because of capacity limitations. Each new batch has at least slight modifications, after all the batches might be several years apart.

We know of at least two Ambassador versions, this points to two batches. Maybe Starfleet didn't need so many oversized ships at the time, and only some 20 or 30 ships were built altogether. In my opinion the Nebula is the Ambassador class successor. As soon as the Nebula was available, no further Ambassador class ships were built. The only problem is the Zhukov registry, but look at the model: the name is misspelled as well ("Zuhkov"), so why trust in the registry?

The Excelsior has to be replaced by a variety of other ships, owing to the large number of Excelsiors. I don't think Starfleet puts one of their classes into a real mass production (hundreds of ships) while there are so many different classes. Since there is a batch limitation, there is no reason to to build batch after batch of a single design, while other designs are available and perform well too. Still, one of the most suited replacements is the Akira in my opinion.