10 at max.
On idle the whole package lies around 272MB(the whole stack with sidekiq, unicorn, redis and postgre, checked on a 1GB box) and I doubt it would rise by much if you have 50-60 users on there.
The only time ive seen the use of 1GB(or close to it) is when its installing.
FYI: My PHP-MySQL setup with 5 php-fpm workers take up 291MB on Idle.
(But that one can be used for multiple forums, yes).