[edited briefly for content April 7]
This is a simple back-of-the-envelope approximate statistical exercise that took less than 10 minutes to do, using the "Sepoy" approach that generally looks at every Indian problem with a non-Indian (western) lens, and measures Indian performance on western metrics. Unlike the Amartya Sen humanities approach, we will not use some wild POTA ("pulled out of thin air") numbers to make up a case, and will try working with plausible approximations using data available in the public domain to make order-of-magnitude calculations.
In predominantly Christian USA (population 314 million) has approximately 450K churches.
Rate: 1433 churches per million of national population.
How many mosques are there in Saudi Arabia?
There appear to be 20K in Mecca alone (population 2 million). wow. But let us suppose than an average, "less-holy" Islamic city perhaps has a rate that is only half as much.
Rate: 5000 mosques per million of national population
Number of dharmic Mandirs in India?
South India has an estimated number of 110K temples. [I assume this number ignores the street-side deities and tiny shrines that doesn't allow more than a handful to congregate - there's probably a couple of million of those in India - and counts only the reasonably sized mandirs that allow public services]. Relatively speaking, this region suffered the least in terms of wholesale temple destruction by Islamic invaders. Still, let us conservatively assume about 100K mandirs per quadrant, which gives us about 400K temples in India (population 1237 million).
Rate: 323 mandirs per million of national population
If India is to achieve parity with just the US rate (forget the middle-eastern rate that is much higher), it roughly needs to increase the number of dharmic mandirs by a 4X factor. To achieve this target, India would need to construct:
(1433-323) * 1237 = approximately 1.37 million more mandirs have to be built. Given that these are rough calculations, we can conclude that an order of a million new mandirs have to be built in India to achieve some degree of "parity" with the US, and several million more to achieve parity with the middle east.
Perhaps more importantly, similar statistics can be compiled to calculate the additional number of Hindu, Buddhist, Jain, and Sikh seminaries and educational institutions, libraries, think-tanks, etc. that have to be built to achieve parity.
Modi's manifesto talked about one temple, and the sepoy media goes into shock. Let them take a look a this :)
The Lost Temples of India
What started off as a ten-minute trivia exercise turns interesting when we ask (assuming that the above calculations are not way off): Why is the Mandir number relatively small? After all. Hindus are/were pretty "religious" like anybody else, and Mandirs were the most important and popular public institutions in the past and its number in India must have been proportional to the size of the population served. My own line-of-thought is to ask: are these "lost temples" partly or largely attributable to the wholesale destruction of temples by Islamic invaders over the last millennium? Descriptions of these acts are available in rich detail via first-hand accounts. If we compare the Mandirs-per-million population in different geographical regions of India and segment these areas into those most affected and those least affected by foreign invasion, and statistically adjust for time, population-growth, and other factors, we may be able to get an order-of-magnitude estimate of the number of mandirs destroyed in this manner. This may help uncover a portion of that shocking era in Indian history that is being white-washed by Marxist historians. This important statistical analysis needs to be taken up by Indian engineers and scientists.