How often can I buy Sudafed?

So basically my whole family and I have a bad case of the flu/rhinitis. I have been to Walgreens and bought 1 box 24 pack and 2 box of 48 pack of Sudafed this month so far. Of course this is the behind the counter product pseudoephedrine. I am not using this medicine for illicit purposes at all. I just want to know if I can buy some more.