Can I increase rent as number of tenants increases?

[Steven Peterson]

About to start renting our current home after our new home is built.  We've determined that for our 4 bedroom, 2 bath house we can get around $1500 in rent in our area.  We've had some college girls, referred by our babysitter, express interest in renting.  It makes sense to me that at a certain number of tenants the rent should increase, but I'm not sure if this is reasonable or common.  For instance, if we have 3 tenants they rent at $1500, but every tenant thereafter increases rent by $250/tenant or something of the sort.  Is this a common practice?  If so, how have you seen this structured/how have you personally done this?