Here's how the UFC buys break down by year (data provided by various Dave Meltzer Yahoo! articles):
YearEventsBuysAverage2007114.9M445,0002008125.1M425,0002009138M615,000201016*9.1M568,750
* - These figures include the "Aldo vs. Faber" pay-per-view event.
The UFC has sold an estimated 2.6M pay-per-views so far in 2011, including UFC 129. That leaves another 6.5M buys needed to overtake 2010.
Should the UFC keep pace with the number of PPVs it produced last year, the company would need to average 600k buys per event.
The current schedule, however, is more likely to fall short of another 16 PPV events. The UFC has officially announced events up through the UFC show in Rio in late August. Unofficially, they are booked through UFC 137, tentatively scheduled for Liverpool, in mid-October.
Assuming one PPV show in both November and December, that leaves us with 10 more events in 2011. (I should note that the Liverpool show would take place a week after UFC 136, making it unlikely to be a PPV show. But for argument's sake, we'll stick with 10.)
Using that figure, the UFC would need to average roughly 660k buys per event, more than they've ever averaged in any calendar year. They would also have to do it without their biggest draw and three champions out of the five "big" weight classes out with injuries for extended periods during the year.