The two simplest statistics for a jam are the number of entries and number of participants, which can be determined directly from the jam pages (on itch.io).
For the purposes of statistics the numbers to take are the officially visible ones on the itch page, immediately after the jam is completed. Aside from the specific jam restrictions that influence the amount of people who can join, there are additional caveats to these numbers, such as:
one VN submitted to multiple jams (either as the same version, or updated version of the original)
not all team members joining the jam officially
itch page displaying a number that doesn't correspond with the amount of visible entries (usually 1-2 entries off, e.g. the page may have 3 VNs visible, but the counter states 4 entries
entries which don't follow the jam rules, but have been missed or are being tolerated by the jam hosts, which can lead to significant differences when counting "manually", e.g. 105 (official) vs. 91 (manual count) entries in NaNoRenO 2020 as appeared in the article What VN statistics tell us by mikey
jams which allow the submission of pages without a link to a playable entry, these count towards the number of entries (at a later stage after the jam period they may indeed feature a playable entry)
over time creators may choose to delete their games and pages, leading to innacuracies when revisiting old jams
for any more nuanced statistics such as determining partial or full completion, and any qualitative features such as inclusion of voice acting, game length and similar, each entry must be examined and/or information gathered from the creator's pages, which will necessitate some level of editorializing and bias (e.g. what constitutes a "demo")
