many billions, me thinks. say some gov. starts buying, to get rid of nasty bitcoin. say they are halfway, 5mio btc, and price is up to whatever, $100 or so. The strong hands, still holding their btc, will see what is tried and hang on. At least I would, till a price of a few thousand $ per btc.
So to shake the tree from the last few million bitcoin, some billions will be needed, I guess.
Then the gov recognizes the've failed and start selling. Prices plummet and the strong hands sell quickly b/c they recognise they've overplayed their hand. The latest sellers before the turndown are happy and buy back their btc or keep their fiat.
From a personal gain standpoint, I would welcome such an attempt by a gov. I would take the ride with half of my btc's.