7人 pronunciation

When you write for example: 私と７人。
Do you read しちにん or ななにん?
Is there a rule or something?
(the same with 4人)

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Both are correct.

We pronounce しちにん or ななにん about 7人.

Mainly in the east of Japan it is pronounced as しちにん, besides in the west of Japan is pronounced as ななにん.

We pronounce よにん about 4人.