According to BOINCStats the number of overall active participants is currently 162,733. We did reach levels of 500,000+ active users but this was in 2016. Personally I think it would be possible to reach these levels again although the overall trend is moving downwards.
I wrote an article about active BOINC users which could be interesting to read: https://steemit.com/gridcoin/@parejan/boinc-active-user-trends
@parejan thanks for the reply and the link to your post. It was very interesting to see how active participants change over time. Although the trend of BOINC users seems to moving downwards it is encouraging to see in your chart 6 that Gridcoin users are on a upward trajectory!
https://steemit.com/gridcoin/@parejan/boinc-active-user-trends