Once you have the search-head configured to search on your indexers, you can look at the actual number of events repartition per indexer.
index=* | chart count by index splunk_server
FYI about deployment with multiple indexers and a search-head
- you only need a single license and define a license-master.
- you can configure your forwarders to loadbalance the data to the indexers
- more indexers means a better loadbalancing and faster searches (map-reduce)
The solution is having just one splunk instance (with many indexers as part of the same Splunk instance or arquitecture) as big as needed with all the data.
I think this is on purpose to avoid having a small license in tons of servers and query them all.
You would be able to add different Splunk instances as Search Peer to your search head. This will allow you to run your query across search peers. See this