Defining a piston simply by the number of metal teeth is a little misleading, as the quality of the metal and general material and construction are far more important factors. If you want to know which piston to use, you can't go wrong with SHS. Their full metal rack pistons are excellent, and can certainly be used in high ROF and FPS setups with no problems whatsoever. I've found them to be very reliable and actually more tolerant of imperfect tuning than partial metal rack pistons. Proper lightening practices still apply, and the metal rack should be firmly glued into place before use. There is an increased risk of pre-engagement, but this is easy to overcome by short stroking and/or using a strong spring (the same way you would with any piston). There's also less overspin.
That said, a good partial metal toothed piston can be just as reliable if tuned correctly. Again, SHS is the best choice in terms of cost vs performance.
So metal tooth count really comes down to personal preference, and of course whether you want/need to short stroke (since the release tooth must always be metal).