Right now the limitation is 10 categories depth and 50 categories in total for each keyword. The limits should be raised.
This relates to tasks T37402 (subcategory search) and T7244 (intersection search).

From https://github.com/wmde/DeepCat-Gadget/issues/13#issuecomment-111508131 written by @jkroll:

Ok, so these are the various limits we have to consider when trying to raise the number of subcategories searched, and how to change them:

• $wgCirrusSearchMaxIncategoryOptions, which limits the categories per incategory: invocation (i.e. per deepcat: keyword invocation). This is currently set to 100 in CirrusSearch.php. Changing it would involve another patch to CirrusSearch.
• maxresultrows limits the maximum subcategories returned by the jsonp interface per request (i.e. per deepcat: keyword). We could change that if needed.
• maxDepth and maxResults in the DeepCat-Gadget. We can change that as needed.
• The number of subcategories which can be handled by Cirrus and Elastic with good performance. This is a soft limit we have to figure out by testing, probably also have to talk to ops.
• GET request string length limit, server-side: about 8k characters. We can't change that unless we change to POST T105328.
• GET request string length limit, browser-side: roughly 2k characters (~150-175 categories) according to @WMDE-Fisch. We can't change that unless we change to POST T105328.