Example:

User wants top pages for WikiProject:Medicine.

This explains the non-adhoc version of how to achieve counts:

Add WikiProject:Medicine as a project modifier to en.wikipedia.org queries on the pageview API. Need to think this through and how it would work on projects like commons and wikidata, but basically queries could ask for /en.wikipedia.org|WikiProject:Medicine/ and get top(1000) and per-project totals. We can get data for other languages by using Wikidata inter-language links. We can assume enwiki is authoritative for this purpose, even though it's not strictly true and we'll miss some articles with this assumption. It's better than nothing and we can improve the assumption when we have better data.

We need to tag pages with WikiProject in pageview hourly so that info is available when we load data into pageview API

FYI, on English Wikipedia at least, it is possible to query what pages belong to a WikiProject via the PageAssessments API (or directly from the page_assessments table in the enwiki database). For example, to get all the pages that belong to WikiProject Medicine:
https://en.wikipedia.org/w/api.php?action=query&list=projectpages&wppprojects=Medicine

This should be faster and more accurate than trying to use category membership or template links. Eventually this will be deployed on other wikis as well.

More info at https://www.mediawiki.org/wiki/Extension:PageAssessments.

@Shizhao Do you have ideas how to handle problems that kaldari bumped? If not, shouldn't you unassign yourself now?