Determining data capacity for an index is a non-trivial exercise.

According to the Splunk documentation1, determining data capacity for an index is a complex task that depends on several factors, such as:

Average size of event data. This is the average number of bytes per event that you send to Splunk. The larger the events, the more storage space they require and the more indexing time they consume.

Number of data sources. This is the number of different types of data that you send to Splunk, such as logs, metrics, network packets, etc. The more data sources you have, the more diverse and complex your data is, and the more processing and parsing Splunk needs to do to index it.

Peak data rates. This is the maximum amount of data that you send to Splunk per second, minute, hour, or day. The higher the peak data rates, the more load and pressure Splunk faces to index the data in a timely manner.

The other option is false because:

Number of concurrent searches on data. This is not a factor that affects daily indexing volume, as it is related to the search performance and the search scheduler, not the indexing process. However, it can affect the overall resource utilization and the responsiveness of Splunk2.