[Arnt]

It varies.

Some ISPs made sure to allocate rather a lot of addresses around 2010, and have room to grow by allocating more efficiently. The ISP where my colo hosts lives used one /30 per customer at the time (which is a fine, sensible strategy, just not one that saves IP addresses). When one of those old customers leaves, the ISP can use the /30 for four new customers.

Another ISP I deal with has already run out of v4 addresses, and some of its customers only have CG-NAT access to IPv4 today. That ISP already has to optimize many things for low v4 address usage.