One mekugi is not adequate for most people. There was an incident in Japan where an iaidoka had forgotten to replace the mekugi and the blade flew out and killed a child.
I'm sure similar things happened in the past but went unrecorded for the history books.
2 mekugi is what most people ought to have in their blades. 1 mekugi is for genuine swords made in the traditional way and used for traditional purposes only, with the utmost care being taken at every step.
It sounds strange, but the bamboo mekugi are far more reliable than the people who wield the blades.