Approximately 300 have been discovered so far. We do not know how big KBO's are, we do not know what they are composed of, we do not know how frequently they collide, we do not know how many of them there are, we do not know how large the largest one is, and we do not know how many of them there are overall, or as a function of size. That is a lot of things we do not know. Because Pluto is much, much bigger than KBO's, and is therefore much, much easier to study. No clear majority of astronomers are convinced either by arguments such as I have given here, or by arguments given by those who think that Pluto is more like a asteroid or a comet than a planet.
Science is frequently a messy business.
Two or more initial conflicting viewpoints may be presented, and only over time are the relative merits of the various positions sorted out and an eventual concensus reached among those who know about and are deeply interested in a particular question. This process is an integral and important part of doing science, and is one of the foundations of the credibility of science in general. The particular question of Pluto's classification as a planet or a minor planet is not particularly earth-shattering. However, in an era when new planetary systems are beginning to be discovered, and with the imminent prospect of technology which will allow us to discover smaller and smaller planets around other stars, the overall question of how to classify objects as planets or minor bodies takes on somewhat more significance. It seems reasonable that we should adopt a classification scheme which gives consistent and rational answers whether applied to our solar system, or to any other star system.