How many plants do I need to start with?

One plant, once it gets going, will be enough if you are just smoking the dried leaf. They grow fairly quickly. You'd have a fair sized plant after a couple of months growth, say, up to 60cm high, with ideal growing conditions and starting with a 26-30cm plant.  Plus you can always start taking cuttings, eventually having as many plants as you want.  Check out the middle plant on the second picture on this linked page...> (the link will open a new window direct to this, scroll up and down to see other pictures). 

If you are keen to try chewing the leaf (and don't want to wait to propagate your own cuttings), you might want to think about getting two plants. This would also apply if you wanted to get more smoking leaf sooner.

Note that when I say something like a few weeks or a few months growth, this does assume the plants are growing. If you receive plants in winter (i.e. in UK or in similar climate and you are not using artificial lights>) then they will grow but quite slowly and you will need to wait until the onset of spring before you see much more significant growth.