It's something like a /48 for each second m^2 of Earth's land surface. Single addresses are not a useful metric, as IPv6 works a lot more with prefixes (groups of addresses, to simplify management and allow things like auto-configuration). The smallest prefix that you would assign to a VLAN with auto-configuration is /64. So /48 (or 65 536 subnets) in IPv6 would be the equivalent of /16 (like 192.168.X.X but global unicast) in IPv4. That is what normal businesses usually get. Home customers should get a /56 of IPv6, that still leaves space for 256 subnets with basically unlimited hosts in them. Not too bad I think.
I never got why they had to make it that large. I doubt we'll ever want to have that many devices with their unique ip. So why not have a simpler system that makes switching easier? Even one octlet more in ipv4 would've sufficed for a very long time, two would've probably been enough forever.
The first is, they were working on this before the Internet was widely used. Back in dialup days, pre-2000, the draft presented in 1998, working on it in 1995 and before surely.
Compared to today's scope and size, this was nascent/early style change, in something which was constantly changing.
They didn't see any contention likely at the time. Why not have all the universities, government departments, and research bodies switch? This was an entirely different landscape compared to today.
So in their eyes, why not make change? It wasn't a big deal, hell back in the early 90s, people were using token ring adapters/networking still in many offices!!
The second thing is, NAT wasn't a thing back then. The computational power was a limiter for large scale usage.
An RFC for NAT came out in 98 I think, and Linux had ipmasq, but that was brand new in the early 90s.
Basically, ipv6 was crafted before anyone had any idea we'd be where we are today.
In fact, the worry about address space exhaustion in the early 90s was due to a lack of NAT, or even the idea that it could be deployed everywhere at scale.
Where would all the compute power for NAT at scale come from?!
So basically, it was crafted with a different viewpoint.
