Old thread, but I figured out a way to do this with metadata.
| metadata type=hosts index=main | stats count AS "Host Count" | eval "Source Index"="main"
| append [ | metadata type=hosts index=other | stats count AS "Host Count" | eval "Source Index"="other"
| table "Source Index","Host Count"
And you just keep appending until you get all the indexes you wanted.
Kind of wonky, but it's fast.
Try this. keep in mind you are searching all event and indexs with this query.
index=* | dedup host, index | table index, host
Hope this helps or gets you started. Dont forget to accept and vote answers that help.