Building a network cluster with PC is not so easy. Back when I was with the school system, I clustered their G3/G4 Mac Labs to make a super cluster of 64+ nodes. Despite the number of nodes, it was not fast because school network was slow itself. Internally within the lab itself, is was a T100, but when it goes out, it connected to a T10 network. So one can imagine how slow that was.
Clustering Pies have bee around for a while, but in this case they hit the same bottle neck - they used the R-Pi 3's wireless as the clustering agent, which is around 56Mbit (about 1/2 of wired T100). Thus they limit this project to 8 R-Pi 3's, calling it OctaPi.
But one can use wired network for faster results and adding more nodes. One just need a T100 8port or larger hub. You just need to edit a few lines in the config to switch it from wireless to wired networking. If you have less Pies, you can use 6 R-Pies for a HexaPi or 4 R-Pies for a TetraPi.
https://projects.raspberrypi.org/en/projects/rpi-python-build-an-octapi Somebody already used an OctaPi to create and break Cryptography encryption.
https://www.raspberrypi.org/blog/octapi/ Thing is, other systems use the same principles to build such clusters, and with Orange Pi and Banana Pi using Giga-bit wired networking (T1000), such clusters using their systems would be much much faster.