This is public data collected from September to December of 2020.
For a total of 34 320 unique active users that posted or commented during these months,
25 563 were active solely on r/dnd,
7 972 on r/rpg and
785 on both.

At the end of 2020, r/dnd had 2 234 415 subscribers and r/rpg had 1 307 575. To give you an idea on why these particular subreddits were chosen, if we look for subreddits with games on the top rankings of ICv2, something like r/shadowrun had only 40 697 subscribers and r/pathfinder only 29 948.

One possible question is if r/dnd can be a gateway for other RPGs as compared with r/rpg. So, I've also looked at the smaller subreddits for specific RPGs to check how many unique users were also active in them. For example, r/dnd and r/shadowrun had 69 users in common while r/rpg shared 124 with that subreddit. Altough it seems users tend to stick with one particular RPG subreddit giving us very small numbers in terms of overlap, r/rpg users seem to be more likely to also be active in other RPG subreddits. A possible exception is r/pathfinder in which r/dnd has 31 users and r/rpg has 23. But for something like r/callofcthulhu the difference is again considerable with 59 for r/dnd and 179 for r/rpg. It does seem that r/dnd in terms of active users is not only on a completely different scale, but also has little overlap with other games.

Reddit itself was chosen because, unlike most active platforms, the data on each subreddit is easily available, anyone can verify it. The code for this little project is maintained by @jogadorsonhador and is available on GitHub.