I usually start with one document and end up working with two, at least in the first draft stage. I don't quite work "from copy", but I frequently make a back-up of the complete contents of my Documents folder in a "Back-up of [DATE]" folder to my external hard disk, which
means I generally have multiple old versions should anything go wrong. Since I don't actually use or work from those documents, I haven't counted them above, though.
(If only a small number of documents have been added or changed, I go for the "e-mail to own address" version of back-up instead. Similarly, regardless of how long ago it has been since my last back-up, if I've made any significant changes/additions to a story, I also e-mail it to myself)
One of the two documents I work from/with is for the story proper, with the scenes in the (for-the-moment) correct order and if some in-between scenes are missing, a short highlighted piece describing what should be in said scene, roughly.
The other document is for loose scenes I intend to work in somewhere, bits and pieces of background information, notes on things I figure I might not keep straight from top of my head, potential plotholes I've noticed but can't immediately solve, random ideas for scenes too many chapters from where I am that may-or-may-not become part of the novel and culled scenes, bits and pieces I figure I might end up reworking and re-adding to the story.
(Mind, the above is for stories where I work with at least a vague idea of where I want to end up and write my scenes more-or-less in order with the story)