You can create as many as you want, however more indexes do not mean better performance. If you keep your data in many different indexes it's rather the opposite, as if you don't specify a specific index in your search Splunk will need to open each index to check if events that you're searching for are in there.
Dividing up data across several indexes is not something you do for performance reasons, rather it's something you do if you want either different periods for how long data will be kept, or different access permissions (for instance user A is allowed to access index X but not index Y, whereas user B is allowed to access index Y but not index X).
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