I was wondering the same thing recently, and kind of assumed that they might be cutting it close, given that they seem to be pushing the iPhone more than other smartphones since the unlimited plan is $10 cheaper with the iPhone. With a little bit of research though, it looks like they're on track to meet their obligations.
From their 2013 10-K filing:
Apple Contract
Our commitment with Apple requires us to purchase a minimum number of smartphones. Since our launch of the iPhone, we have sold in excess of 15 million iPhones and continue to project that we will meet our minimum obligation over the contract term.
Source: http://www.sec.gov/Archives/edgar/data/101830/000010183014000012/sprintcorp201310-k.htm
Granted, 2013 is long past, but I think it's reasonable to assume that Sprint sold and will sell at least the same number of iPhones in 2014 and 2015 as it averaged in 2012/2013 (~7.5 million/year), putting them right around the 30.5 million number that they're rumored to have committed to buying. It'll be interesting to see what their 2014 10-K says, once it is released. I'm guessing they'll have sold more than 7.5 million.