Hello.

We currently have an Enterprise Search Centre in SharePoint 2013 that indexes everything within SharePoint but nothing else.  I'd like to crawl and index 5 million documents that are stored on DFS shares.  Is there any reason I shouldn't just scale out the existing farm to include additional application servers to host the search components?  Or should I create a whole new farm dedicated to search?

My current farm consists of 1 x SQL, 2 x APP and 2 x WFE.

Any feedback or suggestions welcome.

Thanks,
D

As taken from MS documentation ( 

Consider deploying a dedicated search farm if:

Your search environment includes 40 million items or more.
There are two or more farms in the environment.
You want query results to be as fresh as possible.
You are crawling content over the WAN.
You want to implement a different SLA (failover, backup and
restore, etc.) for the search service than other services hosted by
SharePoint .

In addition, up until now I myself have always gone for extending the existing farm, seems easier to accompl

Thank you for the prompt and excellent response, question answered.