You have the ability to target Repeat buyers based on their purchase count.
*The purchase count is based on orders, not items purchased.
Inside editing your campaign you can scroll down to "Advanced Options"
Click (show) to see the Buyer's Purchase Count
The purchase count per buyer is calculated based on the historic order data that is imported during your initial MWS sync.
If you want to send a unique email for "Repeat Buyers" than you would check the 2nd, 3rd, and 4th purchase or more boxes as seen in the screenshot above.
To learn more about Buyer's Purchase count, please refer to this article.