I agree with Richard. But there is also another sense in which racism leads people to underestimate the number of 'whites'. I am thinking here of the practice of counting somebody as unequivocally 'black' if their ancestry is half European and half African, or even 80% European and 20% African. If skin colour were not considered to be socially and politically significant, this would make no sense at all.
Of course there could be all kinds of differences between races, including differences in native intelligence, ability to learn, and so forth. The only significant question is whether there are such differences, and there has never been any decent reason to believe that there are.
Part of the problem here is that people often speak as if "race" is a well-defined notion, perhaps even a notion with biological significance. But it is not.
Yes, I think we do--generally speaking. For this reason, one of the purposes of philosophical interrogation of the concept of "race" must be to undermine it. In my teaching I try to do this where possible, and in ordinary conversation I have been experimenting with either trying to avoid racial terms altogether or using "lighter-skinned" and "darker-skinned" as descriptive terms. These terms, unlike "black" or "white" are comparative and suggest gradations and continuity (which I think accurate to the biological facts of the matter). Ethnic terms like "African" are useful, too, but don't quite bear the same force of inclusion and continuity. Nevertheless, I don't think their use terribly pernicious, except when their use is exceptional. That is, using ethnic rather than racial terms may sometimes still serve to "other," separate, subordinate, etc. when members of other groups are not desgnated with ethnic terms.
There are situations, however, where using the concept of race can serve morally desirable purposes. ("Using," of course, is different from "upholding.") Such cases typically involve using the concept to subvert itself. I more or less agree with Linda Alcoff's concept of "positionality," which acknowledges a certain kind of social reality to the concepts while not implying that they are fixed, natural, or necessary. Understanding this one might, for example, use the concept of "race" as it exists in some context to advance affirmative action and diversity policies, to teach black literature courses, to run anti-racism workshops, to analyze judicial decisions, to criticize housing and banking practices, to prosecute hate crimes etc. Rather than "upholding" race, these uses of it can help to eliminate it.
I do think it true, however, that improperly used--even in these sorts of contexts--using the concept can reinforce the concept. Taking steps to problematize the concept whenever while engaged in these sorts of practices, therefore, is morally advisable in order to minimze or eliminate this result.