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