Maximum desirable masternodes depends on how his plan for Evolution has developed since the initial version of the Evo documents. Stef is correct; Evan wanted a minimum of 3k MNs in order for IX to be robust and secure. In order to get there, he programmed MN rewards to grow gradually from 20% to the current 45% of block rewards. I imagine that he is quite satisfied with our MN count for the present needs of the network.

In the initial version of the Evo documents, Evan stated that more transactions-per-second would be possible with more masternodes. Since each masternode could be a participant in X number of quorums at the same time, twice as many masternodes could participate in 2X quorums, which would increase the TPS throughput drastically. It's essentially sharding. He made one comment somewhere on BCT where he said that he wasn't going with that sort of sharding approach anymore, so I don't know where we stand exactly on maximum desirable masternode count.

AFAIK there is no maximum desirable number for our current network setup, but I could be wrong. I think it's just economically driven at this point (taking into consideration ROI, price, and so forth). People are fine with their share of rewards decreasing by half due to new masternodes if the price of Dash doubles in the process.