1980 Chevy 305 engine and I have wanted to rebuild the motor and I have taken out the pistons to change the rings you have noticed there is a number on each piston what does that mean?

It may have been rebuilt before. It's probably bore number if rotating assembly was balanced to make sure they stayed on the right journals. (weight added/removed from rods) Check to see if weight added anywhere. it could also be a quickie ring change was done & someone wanted to correspond each piston w/it's cap & not regrind crank.