DannoXYZ 

Not so much for removing, as you can get by with one and use the handlebars to hold the fork steady. But you will definitely need 2 to re-install. One to hold the adjustable-cup steady in the proper bearing-preload position. Then a 2nd to tighten down the locknut on top. You can get by with one if you use keyed washers in between, but they introduce some problems and the high-end bikes & headsets never cut a groove into the steerer-tube or use keyed washers for many reasons.