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."