Seeing how sales momentum for Wii dropped dramatically a few years ago, and considering how bad the system is selling right now, I believe Wii will reach a maximum of 110-115 million units world wide.
I'm quite certain that Nintendo could have reached 150 million easily with existing hardware if they had deployed a somewhat different strategy. I don't see the hardware limitations as the main reason behind the dramatic drop in sales. I'd rather say that value compared to price, was the most devastating blow. Initially consumers considered Wii to have a much higher value compared to its competitors. It combined the lowest retail price with the most unique gaming experience. But as prices went down considerably for PS3 and Xbox360, combined with the fact that Wii over time lost it's uniqueness, the playing field changed.
Nintendo overestimated their product, eventually leading to an arrogant pricing of the system. If Nintendo had lowered the price as soon as PS3 and Xbox360 gained momentum, the Wii could have remained the top selling system for a much longer period, simply by denying competitors a fair chance to compete. The year 2009, which I believe was the first turning point this generation, could have looked much different with such a strategy.
The second turning point came when Sony and Microsoft adopted their products to motion control. Nintendo was clearly caught with their pants down, and had too little to put against that. Consumers saw new and sometimes even better motion control for Xbox360 and PS3. Nintendo should have prevented the competition from getting this advantage by including Wii Motion Plus as a standard controller by the time Sony and Microsoft rolled in their motion control.
The third turning point came because Nintendo was unwilling to eat into Sony's and Microsoft's main market. Nintendo should at least have released a WiiHD around Christmas 2010, instead of giving Sony and Microsoft an cheap argument for consumers to buy Xbox360 and PS3. Such an upgrade could have been possible with existing hardware architecture, and not require dramatic hardware changes.
Nintendo also failed with their online strategy for Wii, but I don't think that failure had a major impact on its sales because most die hard online players, had already dismissed Wii because of it's hardware limitations.
Even though Wii must be considered a huge success for Nintendo, I can't help to think about what sales the system really could have generated if Nintendo had stayed offensive in 2009 and beyond, instead of adopting a defensive approach. My conclusion is that Wii never reached its fullest potential.