Well, I am a Greek woman at Ole Miss and I was a recruitment counselor (Gamma Chi) last year. Panhellenic tells the girls they only technically need one rec. for every house. Three should be the maximum just because houses get FLOODED with recs (so think 3 recs per 900 girls...argh). Two is the magic number in my opinion, but don't sweat if you end up with just one. Sometimes it can be difficult to find alumni.
Quality over quantity, a detailed rec from an alumni that glows is worth more than "soso and so is a very nice girl. she is smart. she would be a good Chi Kappa Delt, etc"