I also just noticed this right now, I was going to file a bug report but then I found this topic.
On forums that are not hosted by Discourse , the text is correct because something new with the code changed the badge page to NaN.
Forums that have not updated yet are not affected by this.
For example, the Dark Gaming forum uses Discourse v3.6.0.beta1-dev from this commit (3352cec1bb) and everything is correct.
Another edit: Admins of the site can work around this by modifying the texts js.badges.others_count.other and js.badges.others_count.one but this bug is going to be fixed relatively quickly.