I'd say six to be safe.
Here's a link to a chart for yards to the pound for commercially spun yarn.
Be aware that handspun will usually be denser, sometimes alot denser, than millspun yarn. Depending on how you spin you may get as little as half the amount on the chart per pound. The tighter you spin your singles the heavier the yarn. My worsted spun yarn (spun short draw from the end of a roving or top) is around 700- 750 yards per pound for worsted weight at the lightest (The lightest I will spin, not the lightest I can. With practice you can spin light enough to match the chart but I personally feel that it's worth the extra weight to spin a little denser and make the yarn more durable) and 450 ypp at the heaviest if I want something really hard wearing.
The type of wool you use matters too. Fibers with little crimp will spin up denser than fibers with more crimp. Though more crimp doesn't always mean airier since the more crimps, the smaller each crimp and the denser the yarn. The type of crimp also matters since some wool has a wavy crimp while other wools have a spiral crimp and a spiral crimped wool will be puffier, though most spiral crimped wools are used more for stuffing than spinning it seems.