How Much of the Staffordshire Hoard has been stolen? Nobody Knows.

Thieving artefact hunter (stock photo)

Maev Kennedy, 'Staffordshire hoard of Anglo-Saxon gold reassembled after 1,300 years ',  The Guardian, Wednesday 12 March 2014

For the first time since someone dug a hole in a Staffordshire field and buried a sack holding five kilos of glittering gold, some 1,300 years ago, the entire hoard has been reassembled far from the public gaze in a drab back room at Birmingham Museum and Art Gallery. The Guardian was invited in to see it last week, but the assemblage can only be disclosed today, now that the gold has been safely returned to vaults and secure museum displays.  Its discovery in 2009 by Terry Herbert, an amateur metal detector living on disability benefit in a council flat, made headlines around the world. Archaeologists, who recovered more than 1,500 pieces from the field, believed they had emptied it of its treasures. Three years later farmer Fred Johnson's plough brought more gold to the surface, and a further 90 pieces, including a helmet cheek piece matching one from the original find, were recovered. The count has now risen to more than 4,000 pieces, many of them minute. 

And of course there is no way of saying how many other pieces are currently in private possession.
see for example (in chronological order) the chronicle of mismanagement of this whole affair since the 2009 excavation:

'Blatant, or What?', Thursday, 13 September 2012
'15 detectorists seen in Staffs Hoard field',  Sunday, 2 December 2012
'Staffordshire Hoard Botch-up Revealed' Tuesday, 18 December 2012
'Has Part of the Staffordshire Hoard been Stolen?', Tuesday, 1 January 2013
"More Staffordshire Hoard Treasure May Still be Buried Out There", Sunday, 6 January 2013
'Heritage Action on the fate of the Other Staffordshire Hoard Objects', Sunday, 6 January 2013
'PAS Partners Better Equipped than "Our Boys in Afghanistan"?', Sunday, 13 January 2013
'Vox clamantis in deserto', Sunday, 3 February 2013
'50K for Dumped Grass Cuttings, Slap on the Wrist for Trashing the Past' Sunday, 3 February 2013
'Heritage Action: Recent Nighthawking Activity on the Staffordshire Hoard Field', Sunday, 3 February 2013
'Heritage Theft "Unavoidable": British archaeologist', Sunday, 3 February 2013
'Staffordshire Hoard Field in 2009', Saturday, 9 February 2013
'Continued Theft on Staffordshire Hoard Field: Including from "Security" Firm', Sunday, 10 February 2013
'Protecting the Archaeological Record from Artefact Hunting', Sunday, 10 February 2013
'So, How Easy is it....?', Monday, 11 February 2013
'Unexplored Site of National Importance Looted, is Anyone at all Bothered?', Saturday, 16 February 2013
'Thieves and Whores in British Archaeology', Saturday, 16 February
'The Brownhills Irony and Tekkie Logic', Sunday, 17 February 2013
'Staffordshire Hoard Archaeological Scandal Act Three', Monday, 18 February 2013.
'Nighthawking on the Staffordshire Hoard field: UPDATE (Part one, guest post)', Saturday, 14 September 2013
'Nighthawking on Staffordshire Hoard Field (part two, comment)', Saturday, 14 September 2013

Second vignette: Courtesy Heritage Action, one of the few British heritage organizations having the guts to tell it as it is.