The default setting for the "netusage" parameter is 600 kbps.

Correct me if I'm wrong, but a 100 Mbps ethernet interface is 10 000
kbps (roughly)

Also, the defaut settings for the 10Mbps ethernet is 400 kbps (Ou of a
possible total of approx 1000 kbps)

Does this mean that with these defaults, amanda can not even in
parallel absorb TWO hosts (2 x 400 kpbs) sending their backups at the
same time ?

I would tend to think that a reasonable setting for doing backups at
night time (very low servers usage) N servers with all servers having
AT LEAST 100 Mbps ethernet interfaces would be:

netusage = 7000 (70% of all bandwidth)

And for each of the interfaces, 7000 / N servers ?

Am I right ?