This article states:
To put it in broader perspective, Muslim population has steadily grown from 13% in 1800 to 16% in 1850 to 20% in 1900 to 25% in 1947 and anywhere between 30 to 33% today taking into account the geographical area of pre-partitioned India. These statistics are available from various sources.
It doesn't provide any reference that I can check.
This book estimates that India's population may have been 185 million in 1800 AD.
But it doesn't give any estimate of the Muslim population.
13% of 185 million = 24.05 million. Is it a correct estimate?
According to most widely accepted estimate by historians, What was the population of Muslims in the Indian subcontinent in 1800 AD?
(If such data is not available, I would be happy with estimates of between 1800 and 1850)