I agree with much of what Hugh said. Knowing the purpose to which [or where you plan] you will be putting them in is vital to your estimation of how many and spacing.
Once i bought 300 three year olds, and put them in. i put them in about one every 14\". they all came up, however only have about 20 left now due mostly to deer predation. anyway, i put these in two patches of ground that were 12'x6'. you'd be surprized at how fast they go in , easy to plant, and get used quickly to fill the space. It takes more than you think to do a lot of ground.
I remember that you had a source for two year old roots at a good price. After thinking about it, i would only put in three year olds... I believe that you will have a high mortality rate with younger plants.
Good luck!