Without a concrete measure of the value of a node and a way to determine how many are sufficient or if any number is sufficient, it is impossible to say with any certainty that any particular number is sufficient or that any additional nodes provide no value.

It is reasonable to assume that an additional node provides additional benefit simply because it increases the connectivity of the network. It is not reasonable for you to make your claim (that at some point additional nodes provide no benefit) without supporting it.

Here, I'll help you. It could be argued that an additional node provides benefit at a cost, and the point at which the cost exceeds the benefit is where additional nodes no longer provide a benefit. Now, if you wanted to support your claim, you would need to identify the cost and show how it can outweigh the benefit after some number of nodes.