You could do it with just one pink, one brown, and one white. If you want to use a variety of pink prints and brown prints, you need at least 3 of each -- and more is better so there is more variety. In general, the more variety you have in the prints the scrappier the look (and it tends to become more visually interesting).
How big will your hourglass blocks be? And how many blocks in the quilt? Because I am thinking it will be a pretty small quilt, in which case you don't need too many different fabrics. Are you thinking of buying yardage rather than charm packs or layer cakes?