I am not an expert but I got indexes, sources and sourtypes as well as a custom "Customer" field with the following query:
source=* sourcetype=* [| eventcount summarize=false index=* | table index | format "(" "" "" "" "OR" ")"] |
fillnull value="N/A" Customer
| stats count by index, source, sourcetype, Customer
| sort index, source, sourcetype, Customer
... View more