Here's an article that explores your question of "how many" -
http://www.delawarequilts.com/MyQuilts/Sizes.html
I love sampler quilts, but I think one of the secrets to making a great sampler is the setting. Have you considered an on-point setting? (Sometimes the blocks lend themselves to this, sometimes not.) Or something with special cornerstones or frames around the blocks? If you google "sampler quilts" and then click on images, you will see lots of examples of quilts with all sorts of settings.