It should take about 3-1/2 cans.Some cans will be marked 16oz. Others 14. So the weight is almost irrelevant.
You need to put a thermometer in one of the output registers. Probably the coldest you will get is about 40 degrees.
It is best to verify that the receiver drier is not full of water. There is a drain plug hidden on the bottom of it's tank.
I'd recommend that you also use a system vacuum pump and maybe leave it connected overnight to verify that it holds vacuum before you charge it.
The difference between R12 and 134a is going to be that low temp.
R-12 capable of about 36-7 degrees.