Those numbers indeed look like a guess... for one, for the Benelux (where I live) they give 45.000 Amiga's - compared to 1.5M for the UK. Back in the days *everybody* had an Amiga (or ST) and we had at least one good running national magazine, several big gatherings, so 45.000 seems off to me.
I read somewhere that in the best year, they sold more than a million A500's...
Any pointers for more material?
I know that CU Amiga was selling close to 60,000 copies a month at one point. Try contacting the publisher of the magazine to see if they can provide you with sales figures for some research your doing. I'd say that it's a fair bet that if magazine sales are anywhere near the 20-22,500 mark, more than 45,000 machines were sold.