The advantage of having multiple knifes of one kind is that you can throw them at the target without having to stop. This makes training more efficient. And eventually, every knife (the tip especially) will break when thrown against stone or hard wood occasionally. Most often, the tip will be gone, but some designs are prone to the handle breaking of exactly where it meets the blade. Then it is a plus when you have a replacement for your favorite throwing tool at hand. I suggest you look around and find a knife that suits you. If, after a few training days, you still like it, go ahead and buy 2-3 more of it.
How many throwing knifes do I need?
If you throw in an area with much grass or leaves on the ground, it is best to instantly retrieve your throwing knife if it didn`t stick. Otherwise, if you try to find it later after having thrown the other knifes, you won`t clearly remember where it fell down. I changed to this mode after having lost two good knifes :-(