How many unique people write reviews on Amazon every month?

Define the scope:

When you say unique people, we are not accounting for the # of unique reviews. We just want # of unique people who write reviews and there could me multiple reviews written by a single person for which it will be counted of as 1 unique user only.

Region: USA

Are we considering only direct consumers? Or B2B users as well?

Lets limit it to direct consumers for this discussion.

Formula:

If we find out the # of unique users who order per month, classify them into orders with low value, mid value, high value and orders with faults/damages and % of whom write a review, we could get # of unique users who write reviews per month.

Let's assume:

For Low value orders (say < $20) - 1 / 100 users write reviews

For Mid value orders ($20 - $100) - 5 / 100 users write reviews

For High value orders (>$100) - 10 / 100 users write reviews

For damaged orders - 30/100 users write reviews

Now, US population = 300M (Thinking to be a skewed population divide with life expectancy of 0 - 80, per age group there should be approx 3.75 M people)

Hence, the total unique people who write reviews on Amazon per month are 0.3M +1.27M + 1.36M + 0.96M = 3.89 M users