Around 5 or 6, but It's because I'm developing an american twist serve for my 2nd serve and it just goes short. It still feels awkward, especially because when I bend my knees, I keep my back perpendicular to the ground, and not "straight", if you know what I mean. Just to tell you, my slice serves have better consistency than my top spin ones...
If I ever commit a fault on a flat serve, it's just a bad toss or that I lost my balance when serving, if not, it almost certainly goes in.