We don’t currently have a place on the website you can find this information but you can use our API to search for this data.
This API query shows you the total number of content items registered at Crossref with a breakdown by content type: http://0-api.crossref.org.library.alliant.edu/works?facet=type-name:*&rows=0
When it comes to preprints, these would sit under the heading ‘posted content’. While the majority of the content under this heading is preprints, it is not exclusively and can include things like working papers and dissertations. Also there may be instances where a member registers preprints under a different content type. So it is not a completely accurate figure but should still provide an idea of the growth of this type of ‘posted’, but not yet published, content.