Progenoid are traditionally depicted as producing one set of geneseed per organ, making two per marine, but those numbers just dont add up. Firstly Apothecaries cant get every gene seed from the dead, and often aren't deployed on smaller missions. Lets be generous and say they get half the gene seeds. But then not every geneseed produces pure implants, due to mutation, degeneration and corruption. Then we have to account for the fact that many scouts dont accept the implants and die due to complications, of those that live many scouts are lost before they reach the rank of full battle brother. Once they are marines it still takes 5-10years before their progenoids fully mature...
I think to be realistic you would need 4-5 sets of implants from each progenoid, potentially making 8-10 sets from each marine. That would be much more sustainable.
That point seems to change depending on whose writing the story, it started appearing a few years ago and many continue to write as if they can only be collected from the dead.
If you accept the figure of 8-10 sets of impants, but then deduct training losses, implant failure and dead scouts you get (a very rough average) of about 2-3 actual Space Marines per generation. Which would only allow for a very slow rebuilding of a Chapter.
As I remember correctly the progenoid glands hold the genetic information about the Astartes that carried them. With the collected information the chapter can create the zygotes (also known as gene-seed), which are the basis for each Astartes implant. These zygotes can be created en masse, which is shown by the fact that each and every chapter has to send probes of their gene-seed to the Adeptus Administratum I think. (Not doing this was a start for the Badab War)
I think that the freshly collected progenoids show the qualities of an Astartes in his chapter and can make him immortal by collecting his data. As another aspect they show the current status of the chapter's gene-seed, whether it is flawed or even mutating to a degree one could call heretic.