But on https://stackexchange.com/sites we are on the 6th place:
And I can't find the sort order on https://stackexchange.com/sites page, which fit order on the main page.
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That list of sites is sorted according to what StackExchange.com considers to be the "default" sort. It's a value calculated per site that's a combination of total question and answer counts, answered %, total users, and daily visits, sorted in the descending order.
((TotalQs + (TotalAs / 3)) * PercentAnswered / 100) + TotalUsers + VisitsPerDay
I have no idea why that formula is what it is, but this sort can't be replicated on /sites, which is why you're seeing a different ordering. You can see the sites ordered by this formula on the Leagues page.