Thank you for the suggestions.
I went with your .Count approach and in my For Each …values.* I simply have an If-activity to check whether currentvalue.Path.Contains(“values.document-”).
Perhaps not the prettiest solution I came up with, but it works 
Assign int_NumberOfDocuments = jobj_Blanketdata.SelectTokens("..[?(@.identifier == 'ElementUpload')].values.*").Count(function (x) x.Parent.Value(Of JProperty).Name.startswith("document-"))
If int_NumberOfDocuments > 0
For Each jobj_Blanketdata.SelectTokens("..[?(@.identifier == 'ElementUpload')].values.*") as currentvalue
If currentvalue.Path.Contains("values.document-")
Add currentvalue to list_DocsForExtraction
Regards
Soren