If you hidden blade opponent unit and they sacrifice it how many cards they draw

No — they draw 0 cards from your Hidden Blade. Their own kill/sacrifice removes the unit before Hidden Blade resolves, so Hidden Blade mistargets: the "Kill a unit at a battlefield" instruction is ignored, and since the draw is a linked instruction to it, "Its controller draws 2" is ignored too (CR 359.3.e.14.a; FAQ #3700).

The exception is if the "sacrifice" is actually a replacement effect that keeps the unit on the battlefield (Zhonya's Hourglass, Sett legend). Then the unit is still a legal target at a battlefield when Hidden Blade resolves, so its controller still draws 2 even though the kill was replaced (FAQ #6894).