It was not working alone, so I had to add the basic5: text to the managed property to make it work, as explained here: http://www.sharepointanalysthq.com/2011/01/refining-on-content-type-in-sharepoint-2010/
Number of categories to display I put it in 10.
I changed the xml to this:
<Category Title="Content Type" Description="The content type of the item" Type="Microsoft.Office.Server.Search.WebControls.ManagedPropertyFilterGenerator" MetadataThreshold="0" NumberOfFiltersToDisplay="4" MaxNumberOfFilters="0" SortBy="Frequency" SortDirection="Descending" SortByForMoreFilters="Name" SortDirectionForMoreFilters="Ascending" ShowMoreLink="True" MappedProperty="ContentTypeRefinement" MoreLinkText="show more" LessLinkText="show fewer"/>
I ran a full crawl. After editing the webpart. I try a different few searches but it behaves not normal in my opinion.
For example there is an image called WelcomeImage, if I search for that, 1. Shouldnt I get a Image content type on the left?
If I search the word site: there are many results, but I only see 2 content types, discussion board, and a custom content type. What about subsites? Arent subsite content types?
What about if my keyword matches the name of a list or the name of a subsite?