I don't think the grafting tool matters much.  I have a different one and the number of buds I leave is the same as when I graft with a knife.  I like leaving two or 3 buds on most of my grafts.  Sometimes a bud that looks good does not break bud for some reason.  When I get more than one bud breaking dormancy, I typically keep only one.  It is usually the top most, but sometimes a lower bud will grow with much more vigor.  I wasn't one single leader.  The longer I leave other branches grow, the more they take from that central leader.  
Thanks,
Jack