From "How Many People Are Aromantic?" from the blog The Thinking Aro:
Based on the 2014 Asexual Community Census data provided here:
25.9% of asexuals are aromantic.
If we run with the studies performed on self-reporting asexuals currently on the books, approximately one percent of the human population is asexual.
This means that in the United States alone, there are approximately 3,161,000 asexuals.
The number of aromantic asexuals in the U.S. should be about 818,699.
(The one dilemma we have in arriving at accurate numbers is that we can’t know if the 1% of people who are asexual includes demisexuals and gray-asexuals or not. The 2014 Asexual Census reports that 25.9% of asexuals are aromantic; 9.1% of gray-asexuals are aromantic and 3.5% of demisexuals are aromantic. When dividing the entire asexual spectrum by romantic orientation, aromantics make up 19% total, so if gray-asexuals and demisexuals would be part of the 1% of “asexuals” in the human population, then the number of aromantics would be lower than what I listed above. But if they aren’t part of that 1%, then the numbers would actually be higher.)
Now, the data collected through the 2014 census from sexual people (anyone who isn’t asexual, demisexual, or gray-asexual) is obviously tough to accept as accurately representative of the real sexual population because sexual people who completed the census would obviously be a specific type who has exposure to the asexual community online and therefore is aware of romantic and sexual orientations and information that most people in the world at large don’t have. But just for the hell of it, let’s consider the numbers that come of the data.
Of the sexual people who took the census, 4.3% identified as aromantic.
So if 4.3% of sexual people are aromantic (which might at first seem high but then again, maybe not), that means there should be around 13,456,377 aromantic sexual people in the United States.
Add that to the more reliable stat we have for aromantic asexuals, and there are roughly 14,275,076 aromantic people currently living in the U.S.
The author seems to place the population of the U.S. at 316,100,00, so these (highly uncertain) figures imply 4.5% of population is aromantic.
Again, this is assuming that 99% of human beings are sexual, excluding demisexuals and gray-asexuals. If we were to lump in demisexuals and gray-asexuals as part of that 99% of everyone who is not asexual, then the number of aromantics rises because we add the 9.1% of gray-aces who are aro and the 3.5% of demisexuals who are aro to the 4.3% of sexual people who are aro.
These numbers do not include anyone, asexual-spectrum or sexual, who are demiromantic, grayromantic, or WTFromantic. Those are just the aromantics.
As for whether the aromanticism rate is different among sexual orientations, it is according to Cross-orientations among non-aces from the Asexual Census Blog, which much of the above blog post was based on. The post states that 25.9% of asexuals, 3.5% of demisexuals, 9.1% of gray-asexuals, and 4.3% of non-aces are aromantic. It doesn't break down non-ace orientations though for their individual rates of aromanticism.