Edit: Sendgrid answered my question in this post quite well https://sendgrid.com/blog/where-is-ipv6-in-email/
... so really if you have 1000 IPv4 or 1000 /64 IPv6 it's about the same number of IPs that you can use to spam from.
Well, it’s not? There are 2^64 /64 IPv6s compared to 2^32 IPv4s in total. Unless you meant to say a /64 is as expensive as an IPv4.
This comes from the CIDR notation 192.168.1.0/24 indicating the netblock from 192.168.1.0 - 192.168.1.255.
So the number after the slash corresponds to a network size. The smaller the number, the more bits are variable, i.e. the bigger the netblock.
An IPv4 /24 has 32 (address length) - 24 = 8 bits that are variable, i.e. 2^8 addresses. A /18 would have 32-18 = 14 variable bits, i.e. 2^14 addresses.
In IPv6, the address length is 128. A /64 (which happens to be the smallest routable network size in IPv6) is 2^64 addresses - but because it's easy to get large IPv6 netblocks (typical ISP allocations are /64, /56, and you can often get a /48 with relative ease), for abuse blocking purposes, you generally treat the entire /64 or even /56 as one entity (i.e. you ignore the rightmost 64 or 72 bits).
A /56 is 2^8 separate /64s. A /48 is 2^16 separate /64s.
What he's saying is that one /64 address allocation is as useful to a spammer as a single IPv4 address.
I suggest googling something along the lines of “ipv6 allocation 56 64“ sans quotes.
[Edit: never mind; the HN community seems quite generous with their time today.]