I'm going on some very rough estimates, but they are informed estimates nonetheless (also, the data is heavily biased towards trends in the US). Please bear with me here...
Approximately 19% of Americans play video games on a gaming console (source here
). Approximately 10% of the population is left handed. Rounding the U.S. population to 300 million, we have roughly 57 million Americans playing videogames on something other than a PC (this is important because PCs are primarily mouse and keyboard driven thus I can safely rule this population segment out). If we apply the 10% left-handedness rate to that number, we get 5.7 million left-handed people who play videogames on a game console (a DS is, in this case, a game console). Approximately 105 million current-gen game consoles have been sold in the U.S. as of December 2009 (source - here
). Not counting previous generation consoles, that gives us an almost 2:1 game console to console gamer ratio (1.84:1 to be a little more precise). Also, 43% of the consoles sold were Nintendo DS's. Now, here I have a choice: I can either do some fancy statistical analysis (which I don't want to do) or I can assume that 43% of left-handed console owners have a DS. Doing that, I get approximately 2.45 million left-handed Nintendo DS owners.
That's a decent sized number; let's break it down further.
Taking into account the information provided by CmdrTaco, we are specifically talking about the DSi and a game called Base 10. 300,000 DSi's were sold as of December 2009. That gives us 0.7% of all Nintendo DS's sold are capable of playing this game. That whittles our 2.45 million left-handed DS owners down to 172,000 left-handed DSi owners. I couldn't find any sales numbers for Base 10; I'm not even going to attempt a guess there. That's an okay market size in and of itself, but it would be foolish to assume that 100% of that target market would buy that game. I'm pretty sure not even New Super Mario Bros. has that kind of attachment rate.
In summary, the answer is 42.
This "research" took me all of about thirty minutes so be kind.