What is the logic behind unique customers? Is it customers that have an account and place an order? We have many new customers but only 2 unique customers...
Shows the number of unique customer accounts associated with the store view during the defined time period.
for more info, you can visit magento docs:https://docs.magento.com/m2/ce/user_guide/reports/advanced-reporting.html
customer Id doesn't follow that same pattern because creating when two customers the id values are not consecutive numbers. The best way is to use customer personal or business details to make unique Id