No, it saves IP's as a whole.
If your ISP has 1,000,000 customers and it offers static IPs to all of them that means it needs to buy/register 1,000,000 IPs. That's quite a few.
OTOH, if *at most* half of its users are logged on at any one time (and usually a lot less), then it just rents 500,000 IPs and assigns them dynamically to whoever's online. More efficient. And until IPv6 comes totally online, also *very* important; we're running out of 32-bit IP addresses very fast at the moment, so the more saved, the better.