Two 20A circuits in a single kitchen receptacle - what size box do I need?
Cable clamps are things which, well, clamp the cable to keep it from flopping around where it enters the box.  You must only count cable clamps for box fill if they exist, and if they intrude into the box.   If so, all of them together count as 1 of the largest wire.
Equipment safety grounds must be there. You need to ground stuff; using plastic boxes doesn't reduce the need for grounding - actually it makes grounding more important since the outlets can't ground to the box!  If you have any grounds and you darn well better, then all of them together count as 1 of the largest wire.
Pigtails are free.  At least in a statutory sense; they certainly take a lot of room.   However there is no need to use pigtails if you invest in the better, $3 &quot;Screw-and-clamp&quot; type receptacles, which are able to clamp 2 wires under each screw. Stuffing a recep with 8 stiff #12 solid wires into a box, that'll be fun. Seriously, I'd pigtail with stranded wire, and still use screw-and-clamps (since putting stranded wire on a screw is hard).
Your setup has 2 circuits serving the same yoke.  Those 2 circuits MUST be handle-tied. or on a 2-pole breaker.  As a practical matter, that will oblige you to put them on opposite poles, unless you do something weird with a GE panel.
If you are using GFCI breakers, you might want to acquaint yourself with the Multi-wire branch circuit. This provides a way to do this same thing with only 6 wires instead of 8.  (neutral must be pigtailed, and if you don't pigtail the others, you're only muscling 5 wires).  MWBCs are generally considered archaic because of the need for a 2-pole GFCI breaker, but you're pretty much there anyway, so why not?