Ahmadiyya leadership has always played this game of exaggeration. They lie and exaggerate about everything, simply to please the Mirza family. In this case, I found a newspaper clipping from 1924, this is when Mirza Basheer-uddin Mahmud Ahmad was summoned to the UK, for political reasons, and to discuss his Khilafat, since the British government dismantled the last Islamic Khilafat, i.e. The Ottomans. Ahmadiyya leadership was claiming 500k, as their population, which is an obvious lie, since the British-India census of 1921 proves other wise. As well as the 1931 census.
Nonetheless, here it is, the Dundee Evening Telegraph, dated Monday, Sept-15, 1924.