Warhammer 40k: Do 'decurion' style detachments comprised of multiple formations still count as one detachment for ITC purposes?
“Decurion Style” Detachments comprised of multiple sub-detachments count as 1 detachment towards the 3 detachment limit.

Source: ITC 2017 FAQ and Format Guidelines  (linked from ITC2017 FAQ, Format and Scenarios)

A new edition may render this answer obsolete, but until then, this is the official ITC position.  Including full text for posterity:


  An army may duplicate a single Detachment, one time. 
  
  Example: A list
  may include 2 CADs, but not 3 CADs. Or, a single CAD, and 2 Allied
  Detachments. Or, 3 Formations, 2 of which are the same Formation, etc.
  
  This includes “Decurion Style” Meta-Detachments which are comprised of
  multiple datasheets and Formations. They may only take a single
  duplicate Formation, one time within the Meta-Detachment, although
  they make take duplicate Data Slate units within the Detachment if
  permitted to do so. Example: Necrons may take the Decurion with 2
  Canoptek Harvest formations within it, but no other duplicate
  formations. 
  
  Note: “Decurion Style” Detachments comprised of multiple
  sub-detachments count as 1 detachment towards the 3 detachment limit.