The Splunk Distributed Deployment manual gives a summary of recommended numbers of indexers and search heads at
link text for a number of indexing volume and numbers of search users.
I don't understand the line for 2 to 100 GB/day, which lists one dedicated indexer and no search heads. If there is only a dedicated indexer then surely a separate search head is needed?
You're right, that is confusing. If you're dedicating a box to only indexing, then you will clearly need a separate search head. There is room in that range for a shared box though, depending on the specs and how it will be used.