The following are a few questions that you need to answer in order to determine how many Splunk instances will be involved in your deployment:
How much data will be indexed per day? How much data will be kept? The rule of thumb is 100 gigabytes per day per Splunk indexer, assuming that you have fast disks. See the Sizing indexers section for more information.
How many searches will be running simultaneously? This number is probably smaller than you think. This is not the number of users who may be using Splunk, but the number of queries that are running simultaneously. This varies by the type of queries that your group runs.
What are the sources of data? Where your data comes from can definitely affect your deployment. Planning for all the possible data that you might want to consume can save you from trouble later. See the Common data sources section for examples.
How many data centers do you need to monitor? Dealing with servers in multiple locations introduces another...