In all reality those two blades are probably all you will ever need, it would be prudent to have a couple of back ups, just in case something on the order of an ACT OF GOD or an ATOMIC event goes down, you'll have a spare or two. Your question has nothing much at all to do with NEED but is really has to do with WANT and to lesser degrees justification and perhaps a little guilt and doubt.
I have five knives at current down from say 40+/-, I sold off everything and consolidated to the five Missions I have now, 2 Mission MPF 1 Ti PE, 2 Mission MPS 10 Ti PE, and 1 Mission MPF 3 Ti PE. Have been think lately it sure would have been nice to keep a few of the others for myself to practice sharpening on, to tech my son how sharpen on, to tinker, work on, modify, test, etc.
Now I have five high quality knives that will probably last beyond my sons lifetime, but I don't want to tech him to sharpen with a knife of that quality. So looks like I'll be buying somemore knives.
Moderation is the key don't swing too far in either direction by havign too many (and yes there is such a thing) conversly don't sell off everything and minimize down to too few (and yes there is such a thing) Find a happy median and settle.
MIke C