What religions have the most people/members?

That really depends upon which source you believe. For example, the World Almanac (1999) lists Roman Catholics at 1.04 billion; Protestants, 391 million; Orthodox, 223 million; Muslims, 1.15 billion; Hindus, 747 million; Buddhists, 353 million; Chinese folk religion (includes Confucianism and Taoism), 363 million; Jewish, 14.9 million; Baha’is, 5.5 million; and Shintoists, 2.7 million. It dates the statistics to mid-1997 and credits as its source the 1999 Encyclopedia Britannica Book of the Year.

The World Book Encyclopedia (1999), lists Roman Catholics at 1.04 billion; Protestant, 382 million; Orthodox, 173.6 million; Islam, 1.01 billion; Hinduism, 751.3 million; Buddhism, 334 million; Chinese folk religion, 141 million; Judaism, 12.9 million; Baha’is, 7.67 million and Shintoism, 3.3 million. The World Book figures are from 1993, and are credited to the World Christian Encyclopedia and the American Jewish Committee.