There is no single API call that will fetch ALL of the topics in the category. You will have to make multiple api calls. This is because a category could have 1 million+ topics and that could be a huge query and affect the performance of your site. When you reverse engineer the scrolling you can see how the discourse UI makes the api calls for more topics and you can replicate that behavior in your api.
Depending on what you are trying to achieve you can also use the data explore query to get the number of topics in a category and make an api call to that saved query.