While Raspberry Pi's are awesome, and the power consumption is nothing to scoff at (when considering a cluster), you can accomplish this same thing for a lot less, and have quite a lot more compute power, by purchasing a used server or even something like an AMD 3600X... A single 3600X will grossly outperform this cluster (and cost less) with less headaches (you don't have N physical machines) by using KVM to deploy…
Also, gotta factor in electricity hosts.