For up to 10 concurent users, I use 512MB RAM and 1 CPU, and I host the databse on a shared server.
In my cloud, for this case I have two servers, one running the databse (shared by more instances) which is 2GB RAM and 2 CPUs and the application server has 512MB and 1 CPU. On the application server I run the odoo server and the NginX server that controlls the access.

In my tests I was able to tune the server to go up to 30 concurent users on the small instance, but I don't want the server to be more than 70% utilized in peek.

I use 2 worker threads + 1 cron thread per available CPU, and 1 CPU per 10 concurent users. Make sure you tune the memory limits and cpu limits.

I suggest close monitoring of the server resources (CPU, Memory, Network, etc.) in order to further tune the parameters.