Well John, you are right. You only need a couple of knives and maybe a back-up (or 24 backups which allows you to lose/damage one knife per month and still survive two years
).
I used to think that I was buying knives that had different sets of characteristics and I was searching for the one that was right for me. Then as I started to make progress, at least in finding things I like in knives, I found that I started buying knives that became more and more similar looking in specs. So now I find myself thoroughly in the situation of having duplicate knives.
So far they all get use under rotation but I don't think I'm much closer to ending my knife purchases, even though I have what I need. Maybe I just kid myself that there are still a number of variants that I haven't yet tried out - scandi and convex are two of them. Still don't have a machete. Oh and all those individual makers are so creative in their approaches to handle ergonomics.....
Maybe at some stage I will have a pretty glass case with a bunch of NIB knives stored in them. Personally I hope not. I noticed a lot of folks, good friend around here, have been thinning the herd in the exchange areas lately. Could be the economic times, could be that some have reached their satiation point. I'm not there yet, but I do notice my buying urges are tempered a bit (well not too much since I have an order in for one of those Stomper blades)....
So back to the OP question. All the knives I buy now seem to have a blade length between 3.5 - 5 " and almost universally are .125" thick, except the smaller blades tend towards 3/32" thick. For a single fixed, I find that I like a little bit longer than 4" and a little shorter than 5". Currently, I tend to favor knives with a pronounced belly and that usually comes along with a slightly wider blade. I like a more centered point afforded by a clip or spear point but that might have to do with familiarity.
For the handle, I like something that really fits my hand. That seems obvious, but I have found that erring on the side of caution (overly long handle) is just as bad as too short a handle. For my hand, 3.75" to 4.5" handle length just really works. Finger notches can really augment comfort when holding the knife in a standard grip but can be limiting in other grips. A good compromise seems to be an index finger notch that serves as an integrated guard. Palm swells seem to help chopping and snap cuts, but then they also tend to slide your hands toward the blade when you loosen your grip. Finally balance is important. A great knife seems to have heft that gives you confidence in it, but also naturally orients itself in your hand. The tip and blade edge just do what they are supposed to do without you having to correct the knives position.
Fortunately, I haven't found that perfect knife yet, so I just make do with the ones I have that come pretty close. Unfortunately, there are still a few that come pretty close but for different reasons or different characteristics.
"How many knives do you need???" 
