I selected 300-500 because this, for me, strikes a good balance between have a healthy amount of species, being able to properly represent many different regions and ecosystems, and ensuring that every major exhibit has a chance to shine. That said, the criteria in the OP is vague and doesn't take certain factors into account, so really, any of choices you put could be "ideal" (except for, perhaps, both the lowest and highest answers).
It would largely depend on how much space and resources you have, what kind of animals you plan on exhibiting, how your zoo is laid out (which is more important than people realize), what kind of exhibits you are planning, and what key areas you are focusing on.
Generally speaking, if your zoo is small or focuses primarily on large mammals that are housed outdoors, then obviously that would limit the ideal species count to something lower. However; if your zoo is larger and/or focuses heavily on small animals like reptiles, amphibians, birds, or fish, AND you have the space and resources to comfortably house a larger amount of species, then I see no problem with having a higher species list.
On the subject of large species counts, while I can see where the comments I am seeing in this thread stating that zoos with over 500 species are too "excessive" and "overwhelming" are coming from; as someone who really enjoys small animals and good exhibit design, I find myself easily getting "lost" in well-made and species-rich exhibits, I do feel that species count alone is not a factor of whether I personally find a zoo is overwhelming or not. I feel the zoo's size, layout, exhibitry, and what actually makes up the collection are equally important.
Compare Omaha vs. North Carolina, 2 zoos I have visited.
North Carolina has around 120 species (based on
@nczoofan 's most recent species list). This is leaning slightly towards the smaller side among major US zoos, so in theory, it should not be anywhere near overwhelming. But then there's the fact that NC is huge; nearly 500 acres (and that's only developed land) with relatively large walking distances between major exhibits, and with some exhibits being larger than some entire zoos. With any exhibit being a potential time sink for me depending on what's out, I find myself sometimes being "overwhelmed" or "short on time" because the zoo is so huge, that I have to balance time spent observing animals vs. time spent traveling to the next habitat (I solve this by doing 2-day trips, but that's beside the point).
Omaha, in contrast, has somewhere around 900 species (although how much of that is actually on display vs. behind the scenes is unknown to me). That's pretty big and may seem overwhelming at first, but you have to consider that Omaha is a smaller zoo (around ~125 acres), has a large amount of small animals such as reptiles and fish, and features several large indoor exhibits with multiple mixed-species display. There is more to see at Omaha, but I am also spending less time traveling between exhibits, so as someone whose home zoo is NC, Omaha never really felt overwhelming because of the species count (if anything, it was more so due to me not being able to get enough of some of the fantastic exhibits and animals there).
Basically, you could argue that both zoos have their ideal species counts, despite and due to the various differences between them (I do feel that NC could use more species to fill in gaps in their African collection, but that's an entirely different matter).
I apologize if this went off topic, but in my opinion, people should not focus exclusively on raw species count when describing a zoo as overwhelming and/or steering clear of higher species counts just because they may be overwhelmed. A zoo with 500+ species can avoid being overwhelming if it's well laid-out and uses good space economy.
That said, anything over 1000 species can definitely be excessive unless the zoo has multiple indoor exhibits with huge mixed-species displays, like an aquarium or indoor rainforest.
