The average number of orders placed per customer
Formula
Average Order per Customer = Gross Orders / Customers
Explanation
Pretty self-explanatory - the average amount of orders per customer.
Make sure to read the notes though as the exact definition depends on the report you are looking at.
Notes
When this KPI is used in any Retention reports, the selected Date Range is used to select Customers that made their first order in that period but all Gross Orders until today are used to calculate it.
When this KPI is used in any other reports, the selected Date Range defines all activities to calculate this KPI. So all Gross Orders in the Date Range divided by the Count of Customers that placed at least one order in the Date Range.