1464
When Baldur's Gate 3 came out, Steam's overall bandwidth consumption went from around 18 Tbps to 146 Tbps
(reddthat.com)
From Steam's self-published stats.
Baldur's Gate 3 could not be preloaded and weighed in at 125 gigabytes on disk, so when the game left Early Access at 11am US Eastern yesterday, Steam's bandwidth utilization shot up 8x over a span of 30 minutes. I know personally, I saw my download hit over 600 Mbps across a 1 Gbps fiber connection.
Kudos to the system engineers at Valve. It is mind-boggling that they have built infrastructure that robust.
No more than any other release. This is just notable because it was all at once, which Steam is usually good about distributing, but this time it didn't work out and they took the traffic all at once.
Basically you'd see the same thing spread out over days with a usual steam release and the pre-loading that goes with it.
As for a physical release, I mean, I absolutely would if I could...