We can built a ratio between number of believers to a religion and number of google search. Because there is a relevant between interest a religion and search.
Number of believers by religions:
Christians:
2,100,000,000 adherents - tending to decline in terms of global percentage
Muslims:
1,500,000,000 persons - tending to increase in terms of global percentage
Of no religion:
1,100,000,000 Human Beings - tending to decline in terms of global percentage
Hindus:
900,000,000 adherents - stable in terms of global percentage
Chinese folk religionists:
400,000,000 persons
Primal religionists:
400,000,000 Human Beings
Buddhists:
375,000,000 adherents - stable in terms of global percentage
Number of search results as follow:
Jesus 877.000.000
islam 447.000.000
buddha 161.000.000
flat earth 12.300.000
Calculating the ratio of Number of search / number of believers
Jesus 877.000.000 / 2,100,000,000
islam 447.000.000 / 1,500,000,000
buddha 161.000.000 / 375,000,000
flat earth 12.300.000 / ?
877/2100 = 0,42
447/1500 = 0,30
161 / 375 = 0,43
These results correct our theory as true.
average ratio= (0,42+0,30+0,43)/3= 0,39
So; number of believers of flat earth (about) = 12.../0,39 = 30 769 000 is about 30 millions.