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Well, it's not a "Vocaloid Number" as you say. Miku, Rin/Len, and Luka are all in the Character Vocal Series Crypton. CV stands for (C)haracter (V)ocal and the number is what order they came in. Miku is CV01, Rin and Len are both CV02(because they are two voicebanks in the same software) and Luka is CV03. MEIKO and KAITO don't have numbers because Crypton hadn't started the series for Vocaloid1.

BUT. SeeU has a number SV01 because that's SBS Artech's series. No other vocaloid's contain numbers as tattoos on them.