I think around 200,000 seems pretty accurate going off of lemmyverse.net. I sorted the instances by number of users and it seems like it would add up good. The first five servers were 36k, 19k, 11k, 8k, 4k users. After that it was just a bunch of instances with less than 2k users. But that doesn't mean we can't get it up to 6 million :) Let's keep on welcoming the redditers.