Running NM-B in PVC SCH 40 Conduit from Outdoor Breaker Panel
TLDR: Putting many wires in a conduit requires you vastly oversize the wire.  Better to use many conduits, limiting each to four 14-12 AWG circuits or three 10AWG circuits. 

Wire count de-rate

In one box, you propose twelve 12/2 cables in a single conduit.  That is 24 conductors (grounds don't count).  Because you have so many, NEC rule 310.15(B)3 requires you de-rate the conductors at 45% (per table 310.15B3A) 



and per whatever type of cable you are in fact using on table 310.15B16:



For instance if  you aim to run 20A, and use THWN-2 or USE-2, 10 AWG wire is 40A x 45% = 18A -- not enough, you must go to 8 AWG wire. 

If you're asking "why haven't I heard of this before", it's because the way the numbers work out on 310.15B, the de-rate rules generally don't bother 15-20A circuits with 9 or fewer conductors.  It may be a factor for 30A circuits or larger.  Grounds are not conductors, and neutrals are not counted in 120/240V or MWBC circuits, since they only carry imbalance current.  So any circuit in single-phase or split-phase tends to count as 2 conductors, allowing 4 circuits. 

Conduit fill

Now that these wires are getting burly, you'll need to re-visit the normal conduit fill calculations appropriate to the type of cable you use (jacketed cable like USE-2 takes a lot more space than single wire THWN-2). 

Wet location

Since your boxes are outdoors, everything needs to be listed for wet locations.  You cannot use NM cable. You would need cable listed for outdoor use, e.g. USE-2.  If you use UF or USE cable, go back to 310.15B16 because there's an additional derate, for instance necessitating 6 AWG if using UF. 

Transition to smaller wire

Since you can't stick 8 AWG cable on a receptacle terminal, you will need to transition to 12 AWG wire somewhere.  It could happen in the receptacle box, but it'll take a lot less overall wire wrestling if you do it before that.  However this must be a place where you've split to less than 4 circuits per conduit, bundle, or hole.  Also, it must be inside an electrical box which remains accessible.  You can't put an electrical box anywhere that disassembly of the building is required to access it.  Behind a little door is fine; a panel screwed down is not. 

Better way to do it

The best way to avoid that derating is numerous conduits, so you are not exceeding the nine practical limit for #12 and #14, and the six practical limit for #10.  For instance, five conduits will barely get you by: Two conduits for the four 10/3 and two 12/3 - and three more for the twelve 12/2. You will be full-up at that point, and would need to run more conduit to add anything.  I would consider 8 conduits.  Since you're already doing six, doing two more is no trouble.  You're already there, have all the right tools and supplies, and by #5 you're getting pretty good at it. 

Keep in mind conduits must have a certain spacing in order to count as separate conduits. That is for air circulation, as the name of the game is to cool the wires and conduit.