When designing the number and size of indexes, the following considerations should be applied:

Expected daily ingest volumes: This is the amount of data that will be ingested and indexed by the Splunk platform per day. This affects the storage capacity, the indexing performance, and the license usage of the Splunk deployment. The number and size of indexes should be planned according to the expected daily ingest volumes, as well as the peak ingest volumes, to ensure that the Splunk deployment can handle the data load and meet the business requirements12.

Data retention time policies: This is the duration for which the data will be stored and searchable by the Splunk platform. This affects the storage capacity, the data availability, and the data compliance of the Splunk deployment. The number and size of indexes should be planned according to the data retention time policies, as well as the data lifecycle, to ensure that the Splunk deployment can retain the data for the desired period and meet the legal or regulatory obligations13.

Access controls: This is the mechanism for granting or restricting access to the data by the Splunk users or roles. This affects the data security, the data privacy, and the data governance of the Splunk deployment. The number and size of indexes should be planned according to the access controls, as well as the data sensitivity, to ensure that the Splunk deployment can protect the data from unauthorized or inappropriate access and meet the ethical or organizational standards14.

Option D is the correct answer because it reflects the most relevant and important considerations for designing the number and size of indexes. Option A is incorrect because the number of concurrent users is not a direct factor for designing the number and size of indexes, but rather a factor for designing the search head capacity and the search head clustering configuration5. Option B is incorrect because the number of installed apps is not a direct factor for designing the number and size of indexes, but rather a factor for designing the app compatibility and the app performance. Option C is incorrect because it omits the expected daily ingest volumes, which is a crucial factor for designing the number and size of indexes.