Well, that's actually possible. You can limit the number of backers for any pledge level. So you can have a maximum of 50 supporters backing you to receive a "foo" and another number of "bars" for a different price.
So the usual place where you see an unlimited number of backers is for digital products. See the Amanda Palmer or the Planetary Annihilation: They had limits on any backing that required a physical or personalized goods, but unlimited pledges for digital-only pledges - and there the assumption of "the more I sell, the more I gain" holds true.
So the usual place where you see an unlimited number of backers is for digital products. See the Amanda Palmer or the Planetary Annihilation: They had limits on any backing that required a physical or personalized goods, but unlimited pledges for digital-only pledges - and there the assumption of "the more I sell, the more I gain" holds true.