All of this is explained in the Deploying Odoo documentation, which you should carefully read if you want to run odoo by yourself. See: https://www.odoo.com/documentation/13.0/setup/deploy.html

See also basic optimization of Postgresql: https://pgtune.leopard.in.ua

These values are calculated, from resources available. They are mainly useful with Nginx as a frontend proxy.

Workers and cron-threads are based on CPU cores available. Memory is obviously from Ram. But don't forget to reserve CPU power and Ram to Postgresql and Nginx.

Worker number calculation

Rule of thumb : (#CPU * 2) + 1
Cron workers need CPU
1 worker ~= 6 concurrent users

Memory size calculation

Needed RAM = #worker * ( (light_worker_ratio * light_worker_ram_estimation) + (heavy_worker_ratio * heavy_worker_ram_estimation) )

(above formulas comes from Deploying documentation)

On a small VPS like yours, the formulas must be adapted, assuming you will have no more than one or two users and some visitors on website. In your case, there is no easy standard way to calculate those number, you will need metrics to adapt the formula. But an experienced sysadmin can guess right values quickly.k

Hope this help. Do not hesitate to ask further questions.