I often see and hear this.

Problem:
This phrase is redundant.

Explanation:
The acronym “PIN” means Personal Identification Number, so “PIN Number” means Personal Identification Number Number.

Out of curiosity, I searched with the quotation marks for “PIN number” at Google and got about 2,030,000 matches. Yikes! This is a fairly common problem (at least on the Web).

It seems to me that people who say or write “PIN Number” (or “PIN number”)

• have lost track of the meaning of the individual letters in “PIN”,
• never knew the meaning of the individual letters, or
• are comforted by saying a noun after saying an acronym.

I wonder about the intersection of the population of those who say or write “PIN Number” with the population of those who say or write “ATM Machine” (about which I wrote yesterday). The people in that intersection would be comfortable with sentences such as “I forgot my PIN number at the ATM machine.” — double yikes!

Solutions:
“PIN” or “Personal Identification Number”