Hardware for Oracle and PostgreSQL are different. We used to have big boxes for Oracle with lots of memory, SSD disks and rigid economy of CPU (small number of very fast cores because Oracle is licenced per CPU cores). And that hardware was really efficiently used in consuming nearly all system resources (CPU, I/O, disk space and even network bandwidth).

For PostgreSQL we use commodity servers and right now they are not really efficiently balanced (i.e. on some hosts we don't have enough disk space but other resources are utilized ~30%). So we are working on improving that. But I can't tell you exact numbers of hosts for Oracle or PostgreSQL (which is actually ~x3 from Oracle hosts number :)).