Depends on how you use them...
I generally agree with Rudi- you could probably get away with 10 for the whole kit- less if you use a large wheel for more of the deburring. For my RV-12, I have probably gone through about 12 of the 1 inch variety and still have the tailcone to go. As mentioned in an earlier post for the large wheel, the small wheels also last longer if you run them at an angle to avoid forming grooves. However, when doing this on thin sheets, it will sometimes jump and vibrate if there is no groove and it is held at an angle. I actually like forming the grooves to use for finish deburring as they do a very nice job of deburring both sides at once. However, once they get deep, they start to form a sharp edge and can bind. Hence, I just start a new groove at that point.
In the end, they are relatively cheap compared to all of the other costs of the project so I have not tried to scrimp and save on them!