What hardware attribute would need to be changed to increase the number of simultaneous searches...

https://docs.splunk.com/Documentation/Splunk/7.3.1/DistSearch/SHCarchitecture

Scroll down to section titled, How the cluster handles concurrent search quotas, " Overall search quota. This quota determines the maximum number of historical searches (combined scheduled and ad hoc) that the cluster can run concurrently. This quota is configured with max_Searches_per_cpu and related settings in limits.conf. "