Well assuming genetic drift is a factor and for that reason you can't use all progenoid glands to grow new gene-seed, you can discard a proportion of all harvested gene-seed immediately.
If you're down to ten marines and that's all you've got left - say, one in five of them won't produce viable gene-seed organs when cultured, and all of them have had one set of their progenoids harvested already (and lost in whatever cataclysm) that leaves you with eight.
So you culture those and implant them into neophytes but let's say you have a failure rate in standard, Codexy gene-seed of 2 in 8. That leaves you with six surviving recruits and no failsafe. They're space marines so their training is not for the faint hearted and even with a full Apothecarion (Apothecareless would be more like it if they'd lost all the gene-seed in a microwave lunch/gene-seed freezer mix-up), so let's say you have a 30% attrition rate before they become ready for field duty. That leaves you with four (each still growing two progenoids). It takes between five and ten years for their progenoids to mature, in which time you're either sitting around being forgotten (and leaving people wondering where your fleet of massive battleships crewed by loyal chapter serfs has gotten to in these dangerous times), or you're out fighting.
Out fighting you've got fourteen people in your Chapter and you're going to die, ie, you are doomed.
So you wait. And after another 5-10 years those 8 progenoids have given you another 3.168 marines, because you're doomed either way.
Under ideal conditions, starting with say 50 sets of gene-seed and clone-bodies in which to monitor and control their growth, the AdMech (and presumably any Chapter so equipped) could indeed raise a lot of fresh gene-seed for implantation in a relatively short span, because they can reduce those failure rates drastically, but also because they have stockpiles of gene-seed if something goes wrong.