A good starting point for sizing the hardware is here: http://splunk-sizing.appspot.com/ 
 A Windows Server can send something like 500MB of data a day, depending on that you want to collect (WinEvent Logs, Performance Data, Application Logs). Then you need to decide which retention time you need, if you want to use tsidx reduction, if you have mor than one data volume (eg. SSD for hot/warm storage, HDD for cold), etc. 
 A rough estimation for your case (100 Servers) and a minimal retention time of 30 days would result in a space requirement of 750 GB in total for the index partition. On top of that, you should calculate at least 150GB of space for the Splunk application itself, including possible data models, etc... 
						
					
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