No, generally one round means from a particular initiative count to the same initiative count in the next round. So, the NPC is stunned until the beginning of the PC's next turn.
"Effects that last a certain number of rounds end just before the same initiative count that they began on. (PFRPG 178)"
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So PC wins initiative
PC turn: attempts Stunning Fist.
NPC rolls for Fort and fails. He is now stunned and the PC whomps on him during the PC's turn w/ iterative attacks, FoB, whatever and etc.
NPC turn: stunned, can't act.
PC turn: stun resolves and the NPC can act normally? PC attempts stuns again, succeeds, womps
I guess is it possible to stun lock? supposed 2 PCs are stunning.
PC1: stuns
NPC: can't act
PC2: tries stuns again (does the NPC get a 2nd fort save?, would it just extend duration). or can he attempt a stun on his turn against an already stunned opponent?
the stun from PC one wear off, but PC2 stun still up so he can whack whack whack?
Yes.
I guess is it possible to stun lock?
Yes, provided you have enough Stunning Fists attempts to keep it up and the NPC continues to fail his saving throw.
supposed 2 PCs are stunning.
PC1: stuns
NPC: can't act
PC2: tries stuns again (does the NPC get a 2nd fort save?, would it just extend duration). or can he attempt a stun on his turn against an already stunned opponent?
the stun from PC one wear off, but PC2 stun still up so he can whack whack whack?
Yes, PC2 can attempt to stun an already stunned opponent. The NPC would still get a save, but if successful it would not remove the existing stunned condition. Since the penalties don't stack, a second successful stunning fist will essentially just increase the duration, so the NPC's stunned condition ends just prior to PC2's turn.
Moral of the story. Don't stand between two high-level monks.