Does a 28g 20 bore shotgun cartridge have the same amount of shot load
as a 28g 12 bore cartridge ie is the approxmate number of  (say) shot
size 6 pellets in a 30" circle at 30yards more or less equal?

Hello tchai, I think I can help you with your question. First of all,
the 28g (28 grams) you refer to is the weight of the shot which is
loaded in the shell (or cartridge if you prefer). So, for a given size
of shot, 28 grams determines the exact number of individual pellets in
the shell. Therefore, in the 12 and 20 gage shells you mention each
would have exactly the same number of pellets. It would be possible
also to load an amount of powder into each shell to give exactly the
same muzzle velocity for both 12 and 20 gauge. This being done, you
could get the same number of shot in a 30" circle at 30 yards. The
other things that influence shot pattern density are barrel length and
choke selection. Choke refers to how much the bore is constricted.
Here is a good listing:
http://www.chuckhawks.com/shotgun_chokes.htm
What I am getting at is that it might be difficult to actually achieve
the same pattern at 30 yards that you ask for using two actual
shotguns. Another thing that enters into the picture is that normally
you wouldn't have the same grams of shot in a 12 and 20 gage shell. A
person would generally take advantage of the additional diameter of
the 12 gage and load more shot. Here is a comparison of two standard
12 and 20 gage loads:

http://www.federalcartridge.com/default.asp?pg=27&firearm=5

Scroll down to "PREMIUM WING*SHOKŪ HIGH VELOCITY -- PHEASANT" and
compare the two shells. You will notice that the 20 gage shows 28.35
grams of shot while the 12 gage is loaded with 35.44 grams. So, you
would expect more pellets in your 30" inch circle with the 12 gage.

I believe this answers your question. However, if I have left
something out please ask for a clarification and I will try and help.

Good shooting, Redhoss