These are set in your user settings. For your user, this URL should take you there.
For others, you go here (replacing {username} with your username:
https://community.rstudio.com/u/{username}/preferences/emails
Under the setting "Send me an email when someone quotes me, replies to my post, mentions my @{username}, or invites me to a topic", set it to always.
I think the default behavior is set to "when I'm away", so you won't get an email if you get a reply and you happen to have RStudio Community on your browser.
Another pro-tip, you can adjust your notification settings on a thread by thread basis via the dropdown menu at the bottom of the topic.
Also, note that we use discourse to run this forum. So here are their docs on notification options