The graph below shows in purple the number of parchment manuscripts recorded per half-century, while paper manuscripts are in red. The height of each bar represents the number of total manuscripts. Other people have done this graph before, and done it better.
What's interesting about this is the source of the underlying data: Handschriftencensus.de. Its records are primarily concerned with German vernacular texts, so the graph is less interesting for general book history, but all the more interesting for German Studies. The Handschriftencensus makes no claims to completeness, but it does represent the result of many years of thorough effort by competent experts. As the Handschriftencensus records were never intended as data sources, I've needed to clean up and massage the records into a useable form. In this case, I've compelled vague or multiple datings into a single number, rather than relying only on precise dating. I've included the sixteenth century in the graph, but the small number of manuscripts there does not reflect a decline in manuscript production, but instead only a decline in the number of post-medieval manuscripts that are of interest for medieval German literature.
If nothing else, the graph nicely illustrates the extreme scarcity of manuscripts from the Old High German period, and the sudden rise of paper and decline in parchment at the turn of the fourteenth century. This is already well known, but for an initial attempt to treat a new information source as data,unsurprising results are welcome. Handschriftencensus.de provides electronic records for over 20,000 manuscripts, and there are undoubtedly more interesting results waiting to be found among them.