This question is difficult to answer, partly because different nations record demographic data at different times and in different ways, and partly because some don't even bother. France for one doesn't record its citizens ethnicity, so we don't know how many French citizens are ethnically French. There are estimates of course, so we can know roughly, but not precisely.
But then there's even more problems when it comes to the new world, where the question of whiteness becomes more cultural and less obvious. How do we define whiteness? How do we define non-whiteness? Barack Obama has a black father and white mother, and identifies as, and is identified as, black. It seems in north America whiteness is exclusive, while in Latin America this is less so. 47% of Brazilians identify as white, though most Brazilians have a colourful and diverse ancestry. This, incidentally, is part of why Spain's colonial empire fell apart. It became impossible to enforce the casta system after a few generations of blacks, natives, whites, and others blending their ancestries. So how we're meant to make heads or tails of it is anyone's guess.
For simplicity we're going to just go on self identification and census records. Still, the question remains. What is whiteness? I'm going to assume that whiteness means European ancestry, and so (minus colonies) it stops at the straights of Gibraltar, Bosporus, Caucuses, and Urals. Again, the best we can get is rough estimates. If we go on the largest ethnic groups, including significant white minorities, then it seems that around 90% of most EU states are white, while Russia is 80% white.
So, this would mean that given the EU's population of 510m, plus Russia, Ukraine, and Belarus' combined of about 200m, assuming Russia 80% white, while Ukraine and Belarus 90%... leaves us with a grand total of about 624m white Europeans.
Outside of Europe, from the populations of former colonies, this data suggests the total white population is about 480m. So the total white population of the world is roughly speaking 1,102m.
Now, I am going to assume that "white women" basically translates as "females of majority European ancestry". And I'm going to assume a 50-50 gender split, which isn't going to be exactly true, but close enough. So in that case we're left with a world population of white females of roughly 550m. Out of a world population which is presently 7,400m. This means that we can estimate that 7-8% of the world's population are "white women".
In conclusion and according to the stated assumptions: almost double the initial figure, but still a very small group against the world population. And yet to imply five hundred million people aren't numerous enough to support "diversity" (of what?), I don't know. That's still a lot of people, even if a small proportion of the whole.