> how many fills do u get from a 50lb bulk tank
> 
> if using a  9oz tank
> if using a 12oz tank
> if using a 20oz tank

Depends on a lot of stuff.  Mostly how much do you (and the tank) waste.

(16 ounces = 1 pound)

In theory, you can fill 50 x 16 = 800 ounces off a 50lb.

I had a lengthy discussion with my local gas distributor and the siphon
tubes they install are usually several inches from the bottom of the
talk.  They say they do that to prevent it from sucking up dirt, oil,
and rust dust from the bottom of the tank, which would make a mess
(internally) of whatever you were supplying with CO2.  I believe there
is a law against operating a compressed gas cylinder upside down, plus
its downright dangerous, so you see a siphon is going to be required.
Expect to lose about 10% (160 oz) in the bottom, out of reach of the
siphon.

Next important factor is your fill technique.  I have a 20# at home I
use for my tanks, but I chill them in the freezer before filling, and
I never vent the CO2 needlessly or shoot them dry beforehand.  If you
can chill them before filling, you will not need to fill once to chill
and then a 2nd time to actually get a topped off tank.  Ths saves maybe
2-4 oz/fill, depending on the size of the tank.

Lastly, your fill station.  Thin pipes with minimum distance are best.
There are a few stations that attach directly to the tank, and there
is no hose.  This wastes the least gas.  Long wide hoses waste a lot.
(maybe one ounce per fill)

So if you can get at 640oz, and are filling 20oz tanks, wasting 4 oz/
fill, (with chill filling) expect maybe 26 fills.  If you can pre-
chill them, that's an additional 3 or 4 fills. (assuming that wastes
only one ounce)

The siphon usually is the same distance from the bottom regaurdless of
tank size, so my 20# is wasting a greater percentage of my fill, but it
sure is lighter to lug around, and fits better in the back seat. :-)
You may have to "specail request" a smaller siphon tank for CO2, they
are not likely to have a 20#S on-hand, but they will make one up for
you if you ask.  It costs me $13 to fill my 20#, and $18 to fill a 50#.
I lease the 20# tank for $13/year.  If you shoot much, it makes a lot
of sense to have your own 20#.  Oh, fill stations are around $35.
Don't try to build your own, they are more expensive to build than buy.
(usually)