Kindly need your advise, I need to setup indexer cluster on my customer env, but unfortunately they don't have any other server to act as manager node server, my question is, can I enable manager node role on search head server?
is there any impact to search head function?
sorry but you cannot!
as you can read at https://docs.splunk.com/Documentation/Splunk/8.1.2/Indexer/Systemrequirements, it's written in Splunk documentation: "Each cluster node (manager, peer, or search head) must reside on a separate Splunk Enterprise instance."
You have to find another server.