How many digits for the serial number for iPhone pro

Apple announced (early 2021?) that it would be changing to randomized

strings for serial numbers and such strings would be between 8 and 14

characters in length.

Quick search, 1st link https://www.makeuseof.com/apple-randomized-serial-numbers/

YMMV

My iPad Pro 11 has 12 digits in the serial number. But our iPhone Pro 13 and 13 Pro Max, both have 10 digit numbers.

12, why?

My iPhone 13 Pro Max has a 10 digit serial number. Doesn't answer why the OP wants to know how many digits are in one, but I'm thinking 12 is not necessarily correct for all iPhone models.

Ah really?

Its weird all my phones and ipads have 12 but your 13 has 10

Must be a country thing!

Well spotted! How random!