I recently came across a map which displayed the number of adherents to the sect in every country. What surprised me was the world spanning distribution of the faith. How did an initially Indian-Pakistani movement come to gain followers around the world?
I can only speak in very general terms but one of the reasons is that the Ahmadiyya face significant persecution. Somewhat like the Bahai, as a result of the fact that they follow a religious leader who claimed to fulfill messianic prophecies, the Ahmadiyya are rejected by many mainstream Muslims from the faith altogether. They also have a highly centralized leadership structure that promotes international proselytization.
That being said, I would be very skeptical of the kind of figures claimed on maps like the ones here: https://en.wikipedia.org/wiki/Ahmadiyya_by_country
Frankly, I flat out do not believe that there are more than 2.5 million Ahmadis in Tanzania and Nigeria.