I'd say it would be hard to predict the amount of supplementation needed to thwart deficiency, especially in the specific terms you're looking for. Defining the amount of B12 depleted by nitrous would likely take an extensive number of clinical trials to even roughly define the effect on B12 levels. Defining it to such a fine degree that would allow estimation based on an individual cartridge is highly unrealistic. Even if a single charger could necessarily translate to direct destruction of a defined amount of B12, it would vary widely between individuals based on different metabolic rates and established B12 levels.
B12 deficiency takes some time to set in anyway, so whether you take B12 pre or post nitrous is irrelevant.
Oh, and regardless of the fact that we don't use "SWIM" here, it would hardly be necessary in this context anyway, considering that nitrous is legal to obtain and use recreationally in most places.
Lastly, I would say it might be helpful to place a greater focus on moderation, rather than trying to strictly define the amount of nitrous you can safely consume. In any case, unless you're planning to make nitrous a hardcore much-of-the-day/most-days habit, you probably shouldn't worry about.