Both of your questions are excellent questions. Unfortunately, we don't have good answers for them.
Regarding the number of R users, we've been trying to get a handle on how we might estimate that number in a statistically defensible way. Direct measurement of the number of users through techniques such as downloads is quite challenging due to the wide variation in computing environments and what constitutes an R user. Surveys don't really do the job because we have no good way of knowing the proportion of survey responses to the full. population.
When we think about indirect measurement, the education group here at RStudio has some thoughts about engaging some ecological researchers, who spend a lot of their time estimating populations of flora and fauna. At this point, we haven't yet engaged with any of those researchers, but we hope to in coming months.
We also don't have much data regarding the distribution of R and RStudio users. Yes, we do get some Google analytics data from RStudio downloads, but location information from IP addresses is notoriously unreliable due to firewalls, VPNs, and NAT routers that hide end user addresses.
With all that said, I conducted a fairly large survey of R users last December and intend to run that survey again this coming December. You can access last year's raw data and analyzed results at https://github.com/rstudio/learning-r-survey . You can also watch my rstudio::conf 2019 talk on the data at https://resources.rstudio.com/rstudio-conf-2019/the-next-million-r-users. That data doesn't directly address the challenge of counting R users, but it does provide some demographic and country-level distribution info for the survey respondents. However, as I noted above, we don't know how representative our survey respondents are of the larger R user population.
Please let me know if you make progress on these questions -- I'd love to know the answers!