As noted, the limit is, in most cases, 4 of them in all (One per organic foot, one per organic hand) ... this could change in certain instances (If you had more arms or legs, for instance) but, barring that, there's your top-end.
They're done that way for how combat is abstracted, since a 'melee attack' can be any number of punches or kicks, including, but not restricted to, "One punch to the face".
The single one taken is going to be rare, but, it fell into the general design I wanted in there, which was to list augmentations in single-form, but noting that they're usually taken in pairs, rather than having to break out single applications later. (For instance, taking a single cybereye isn't possible in core, which is silly.) ... so, I'd slip a small bonus in most for when you take pairs, but allow for you to take singles if you wanted.
My apologies for not making it more clear in the text. The errata team'll make it official later, but, figured you'd like to know while I'm passing through.
