Ask HN: Postgresql connections per core

How can i to estimate the upper connection limit per CPU core ??? I have read several posts where the sysadm.... recomends..... 2 connections per cpu core (before the disk will be bootleneck...). in another hand.... i can read in several CLOUD postgres services... (SAP for this case) where they sell.... (1 CPU core, 2GB RAM.... with upper limit of 20 connections.....). where is the trick ?