9 of 10 on 7 speed freehub
If you use a 10-speed cassette, you'll need to use a 10-speed shifter as the spacing between cogs needs to match the amount of cable pulled by the shifter. If you want to use a 9-speed shifter, then use a 9-speed cassette and take out one cog. Also, You probably don't want to take off the smallest cog since it's the one designed to mesh up with the lockring. Any other cog (including the largest) should be fine.