I've had 4 villagers die but only two are showing up in the graveyard? Why is this?

Is it possible that you did not bury them? As the bones lie on the ground and will decompose over time. Maybe you did not discover the graveyard in time? Or I believe they will also decompose while the game is off.

The bones were there when I started the game but I wonder if I interrupted them too soon during the burial and so they aren't appearing. They must have dropped the bodies. :P

Yes, if you interrupt villager when they're carrying anything, they will drop it and not complete the task.

Your villagers take the dead to the grave and sometimes you have to show a villager how to pick up the dead. Just pick up a villager and put him/her on the dead and they should bury it for you.

That's not how burial works - once you solve the puzzle of the graveyard, any adult villager will know what to do without your help!

hmm dropping a villager on a dead body will get you some interesting responses from the one dropped - usually will see their action as "Embracing" then see at the bottom of screen "THAT VILLAGER IS DEAD!!"
Some people have reported that when they drop a villager on a dead body that the dropee tries to heal them. I usually take my seniors and pick them up and put them to food bin to keep from watching them walk so painfully across the screen to bury a body - let the youngsters bury bodies.

On the missing bodies - I have noticed if you dont let the villagers carrying bodies get at least half way to the graveyard, then the headstone wont come up and the body they are carrying kinda "disengrates"

Maximum population for the graveyard?

50