Update: I can't answer as the question is closed (interesting enough I have received a badge for the popular question for the question that was closed). But I have found new info that Big Sur doesn't send to the web sites in UserArgent that it is the version 11.00 as Apple expected problems with web site compatibility. As such there can't be any data originated from the web sites and as most currently published data originates from that source it is likely that there won't be any data on this.
I can't find any data on Big Sur market share among Mac versions.
The only source that covers this is https://gs.statcounter.com/macos-version-market-share/desktop/worldwide
However on that source there is no data for Big Sur - if you download .csv you will see that all values add up to 100% and there is no 'Big Sur' or 'Other' category. I would assume that Big Sur has similar to what Catalina had in this period of year (41%) and that all other values should be corrected by multiplying with 59%.
But that is just a guess. Does anyone have any data? Including from his own site if nothing else just if it has at least 1k Mac users a month?