As others mentioned, I really doubt any exact numbers will be given, the admins have always been clear that they do not indeed to give out the exact number of pets on CS (in terms of, like, the exact number of a specific pet, like saying how many plains zebras there are on the site) and this would probably fall into that category.
That said I also wanted to mention that it has not always been possible to adopt the zebras from horse litters. It was for a while when they were first released, and at a certain point that was stopped and they were no longer released through monthly litters. It was eventually brought back and (as far as I know) they are still actively being released in monthly litters (granted at an extremely small probability of adopting one). But just to be clear that they have not /always/ been released that way and there was a period in which they were not which should be factored in. I know someone dug up the dates at one point though I don't have them on hand at the moment, I could probably find them with some digging.
edit; found a post where I'd talked about it some here and ZEL added some clarification as well;
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