Flaw in the API
The API documentation on Posts->Lists->limit it says hard limit of hundred
But in reality it is limited to 21
https://konachan.com/post.json?limit=0 gives you 21 posts instead of 100
I am working with JSON here

Also, the posts are sorted based on the upload date. A Quality of Life improvement would be a random option.
That's a typo or outdated info. The hard limit is 1000. If you try to request more, you will only get 1000.

limit=0 does not return the hard limit, nor is it supposed to. Specifying a limit of 0 is the same as not specifying any limit. It returns the default limit (ie. the same number of posts you get per page).

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You can't expect any changes to the API here. You will encounter some issues, but you'll just have to work around them.

You may be interested to know that the xml version will provide you with a total count of the posts in your search. It's right at the beginning of the result and easy to extract. If you only need one or a few random posts at a time, you can use that to generate your own random list and retrieve additional info for each post individually.