Splunk recommends at least 800 IOPS, but all you can have it's obviously better!
for more infos see at https://docs.splunk.com/Documentation/Splunk/8.0.0/Capacity/Referencehardware
as I said Splunk requires at least 800 IOPS, but if you have to index 1TB/day: storage is usually the bottleneck but to index much data you have to use many CPUs and rAM.
the question is: how many Indexers you have to use?
Remembering that Indexers work also to answer to searches (users and scheduled).
So to index 600-1000 GB/day you should use:
If you have more than 1TB/day, capacity planning grows, 1-2 TB/day:
Then is also relevant how many scheduled searches you have running, or if you have many apps like Enterprise Security or ITSI that contain many scheduled searches requires additional resources.
A complete training about capacity planning is one of the themes of Splunk Architect Training (that I hint!).
Anyway returning to your question: Splunk requires at least 800 IOPS or more (obviously better!), that means 8x15K rpm SAS drives in RAID 1+0 configuration or SSD disks.
Then analyze your requirements and plan the correct number and resources of your Indexers (and Search Heads).