Multiprocessor and 20.04
As ubfan1 suggested, the boot option "acpi=off" was the reason of the non-detection of the multiple processors. Without it, they are back! But the reason why I had added it was the control of the sleep mode: I wanted to avoid the server to enter in sleep mode after a few minutes of inactivity. It has been finaly achieved, without the "acpi=off" boot option, by modifying /etc/gdm3/greeter.dconf-defaults as suggested in the thread Ubuntu 20.04 server suspends or goes to sleep after a certain period of time, causing ssh to time out