Numerically, I'm after Vi (5)

I believe the answer is:

seven

I'm a little stuck... Click here to teach me more about this clue!

'vi' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Perhaps you can see a link between them that I don't see?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for seven that I've seen before include "Prime number" , "Number (of stars in the sky?)" , "- Against Thebes (Aeschylus)" , "So many ages of man (AYLI)" , "Numeral" .)

I've seen this clue in the Evening Standard.

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