As you all know that Hybrid Delve and Search are coming to SharePoint 2016 and SharePoint 2013. Hope below additional details can be of help.
If you plan to use Hybrid Delve and Search feature then you will have to create a Cloud Search Service Application. As far as I know key source behind Delve (Office Graph) for Delve won’t be available On Premises SharePoint.
I see a lot of concern where people ask if the crawled content would remain on Cloud server or On-premises SharePoint server. The content itself remains On-Premises and the indexed metadata is unified in a Cloud Index in Office 365. This mean crawled content
resides in your environment but their index resides on Office 365 servers.
Now another concern is what happens to sensitive data which will be crawled. Its indexes would move to cloud which may not be accepted to everyone.
For sensitive content it would be recommended to not index this content using the Cloud SSA. The Combining Hybrid search approaches can use the current Query federation approach for the hybrid scenario should this need to be queried from SharePoint Online.
Bill Baer says “I wouldn’t approach the new hybrid search experience as sending the entirety of your content to the unified index in SPO, just the “indexed” content, I.e. the first 100K – as related to selectively sending content to the unified index, yes,
that can be achieved with the new hybrid cloud Search SA – the number of servers on-premises as related to scale-related considerations is dependent on your own throughput needs, but can be as few as a single server running either SharePoint 2013 and/or SharePoint
2016 which can serve as a proxy to legacy content sources such as SharePoint 2010. Since the index is unified in SPO and the results returned by Office 365 you can also potentially reduce the scale of your existing search architecture and let the service do
the heavy lifting of storing and returning results.”
To summarize, if you have lot of content which does not contain sensitive information and your company is ready for Hybrid approach you can crawl Hugh amount of data but your search architecture would be very small because half of the load(Index and Query)
would be taken by Office 365 servers. Here you will get a very good search performance even if you have single SharePoint environment.
You need On-Premises SharePoint environment and Office 365 subscription to use Cloud search service application.
Hope this will help you with your SharePoint 2016 planning.