Hani Tahir has explained this already. MGA was lying about the amount of Ahmadi’s. Just a year before this, MGA admitted to only having 313 Ahmadi men with him. Just a year later, MGA claims to have gotten 7700 converts to #Ahmadiyya, which is a farce. Read our essay wherein we give all the Ahmadiyya census numbers.
Ishtiharata (25-1-1897)

– Mirza said: “Shaykh Ghulam Dastaghir is fond of Takfir. So I preach to him that the number of my group after the Mubahalat of Abdul Haq al-Ghaznawi has reached 8000 thousands person… for this reason.” 

The scan

_____________________________________________________________________________________________Links and Related Essays

Mirza Tahir Ahmad announced 40 million converts to Ahmadiyya from India alone in 2001

Mirza Basheer-uddin Mahmud Ahmad admits that Ahmadis lie about their membership numbers (1950)