Hello Robert, to my understanding the answer is 'very few'; I believe the general apprehension during the late twentieth century (perhaps since disproved/ one or more of the families has died out) was that there were only three families with proven descent dating to before the Norman Conquest: Arden (of Arden, Warwicks.), Berkeley, and Swinton, with the last being the only one still in possession of its estate, Swinton, Berwickshire.
Even other notably extremely ancient families such as, for example, Fulford (owned Great Fulford at Dunsford near Exeter since Richard I), have apparently 'only' owned their estates since post-William the Conqueror. Other similar families of this type: Dymoke of Scrivelsby, Lincs. (Hereditary King's/ Queen's Champions) and Giffard of Chillington, Staffs. ... perhaps a few more? That's pretty much as far as I go; hopefully someone with specific knowledge can chime in!