Exactly enough to fill out the address, which is always the same length. BTW, IPv4 does basically the same thing. The address 127.1 is equivalent to 127.0.0.1.

There are a total of 8 groups of 4 hex digits, so 8 minus however many groups you already have.

google.com: 2607:f8b0:4009:819::200e (5 groups) -> 2607:f8b0:4009:0819:0000:0000:0000:200e (3 groups of added zeros)

a ULA address: fd2a:1::2 (3 groups) -> fd2a:0001:0000:0000:0000:0000:0000:0002 (5 added)

localhost: ::1 -> 0000:0000:0000:0000:0000:0000:0000:0001

However many are left. In what circumstances do you care?

However many it takes to make the whole A::B number exactly 128 bits long.

“Enough”