As you notice above that the Address bus saturation causes
poor scaling! And the Address Bus carries requests and
responses for data, called snoops, and more caches mean more sources
and more destinations for snoops that is causing the poor scaling.
So to solve the problem of poor scalability above, you have to use Directory-based coherence that is a mechanism to handle Cache coherence
problem in Distributed shared memory (DSM) a.k.a. Non-Uniform Memory