This thread here describes the issue quite well.
This was my experience with a in match football forum before issues kicked in:
Digital Ocean
1 CPU 1 GB = 30 -40 users in chat like situation
2 CPUs and 2 GB= 70-80 users in chat like situation
4 CPUs and 8 GB = fine for 120 users and 1000 posts in 2 hours. Didn’t reach limit.
With Hetzner (Mirroring site)
My experience was:
3 CPU (CPX 21 AMD chip) and 4Gb = struggling with 20 users
2 CPU (Intel) and 8gb = no issue with 20 users.
I never got around to testing with more users.
The key thing is to improve the CPU and Ram. AND ALSO edit the app.yml file.
Add more unicrons in here, and also alter db_shared_buffers .