I was wondering how many religions have a requirement to observe a vegetarian diet.
Most Indian religions have philosophical schools that forbid consumption of meat and Jainism institutes an outright ban on the same. Consequently, India is home to more vegetarians than any other country. About 30% of India's 1.2 billion population practices lacto vegetarianism, with overall meat consumption increasing. The per capita meat consumption in India in 2002 was 5.2 kg, while it was 24 times more in the United States at 124.8 kg. Meat consumption in the United States and India grew at about 40% over the last 50 years. In 1961 Indian per capita meat consumption was 3.7 kg, while the US consumption was 89.2 kg. (1 kg = 2.205 lb)
In the same WIKI I found the following very interesting, which seems to expose a contradiction between the Pali Canon and certain Mahayana Sutras:
The First Precept prohibits Buddhists from killing people or animals. The matter of whether this forbids Buddhists from eating meat has long been a matter of debate.
The first Buddhist monks and nuns were forbidden from growing, storing, or cooking their own food; they relied entirely on the generosity of alms to feed themselves, and were not allowed to accept money to buy their own food. They could not make special dietary requests, and had to accept whatever food almsgivers had available, including meat. Monks and nuns of the Theravada school of Buddhism, which predominates in Sri Lanka, Thailand, Cambodia, Burma, and Laos, still follow these strictures today.
These strictures were relaxed in China, Korea, Japan, and other countries that follow Mahayana Buddhism, where monasteries were situated in remote mountain areas and the distance to the nearest towns made daily almsrounds impractical. There, Buddhist monks and nuns could cultivate their own crops, store their own harvests, cook their own meals, and accept money to buy anything else they needed in terms of food in the market.
According to the Vinaya Pitaka, when Devadatta urged him to make complete abstinence from meat compulsory, the Buddha refused, maintaining that "monks would have to accept whatever they found in their begging bowls, including meat, provided that they had not seen, had not heard, and had no reason to suspect that the animal had been killed so that the meat could be given to them". There were prohibitions on specific kinds of meat: meat from humans, meat from royal animals such as elephants or horses, meat from dogs, and meat from dangerous animals like snakes, lions, tigers, panthers, bears and hyenas.
On the other hand, certain Mahayana sutras strongly denounce the eating of meat. According to the Mahayana Mahaparinirvana Sutra, the Buddha revoked this permission to eat meat and warned of a dark age when false monks would claim that they were allowed meat. In the Lankavatara Sutra, a disciple of the Buddha named Mahamati asks "[Y]ou teach a doctrine that is flavoured with compassion. It is the teaching of the perfect Buddhas. And yet we eat meat nonetheless; we have not put an end to it." An entire chapter is devoted to the Buddha's response, wherein he lists a litany of spiritual, physical, mental, and emotional reasons why meat-eating should be abjured. However, according to Suzuki (2004:211), this chapter on meat-eating is a "later addition to the text....It is quite likely that meat-eating was practiced more or less among the earlier Buddhists, which was made a subject of severe criticism by their opponents. The Buddhists at the time of the Laṅkāvatāra did not like it, hence this addition in which an apologetic tone is noticeable." Phelps (2004:64–65) points to a passage in the Surangama Sutra which implies advocacy of "not just a vegetarian, but a vegan lifestyle"; however, numerous scholars over the centuries have concluded that the Śūraṅgama Sūtra is a forgery. Moreover, in the Mahayana Mahaparinirvana Sutra, the same sutra which records his retraction of permission to eat meat, the Buddha explicitly identifies as "beautiful foods" honey, milk, and cream, all of which are eschewed by vegans.
In the modern Buddhist world, attitudes toward vegetarianism vary by location. In China and Vietnam, monks typically eat no meat (and with other restrictions as well—see Buddhist cuisine). In Japan or Korea some schools do not eat meat, while most do. Theravadins in Sri Lanka and South-east Asia do not practice vegetarianism. All Buddhists however, including monks, are allowed to practice vegetarianism if they wish to do so. Phelps (2004:147) states that "There are no accurate statistics, but I would guess—and it is only a guess—that worldwide about half of all Buddhists are vegetarian".