Final answer:
To increase the number of simultaneous searches on a single search head, you would need to change the processing power and memory (RAM) of the hardware.
Step-by-step explanation:
To increase the number of simultaneous searches on a single search head, the hardware attribute that needs to be changed is the processing power. A more powerful CPU can handle and process more search queries simultaneously, allowing for an increased number of ad-hoc and scheduled searches. Additionally, increasing the memory (RAM) can also help improve performance by providing more space for storing and manipulating data during searches. It's important to consider both the CPU and RAM when aiming to increase the number of simultaneous searches.