ALL FROM WIKIPEDIA:
Christianity 2.1 billion (see below)
Islam 1.3 billion (see below)
Non-Adherent (Secular/Atheist/Irreligious/Agnostic/Nontheist) 1.1 billion
Hinduism 900 million (see below)
Chinese folk religion 394 million (see below)
Buddhism 376 million
Primal indigenous ("Pagan") 300 million
African traditional and diasporic 100 million
Sikhism 23 million
Juche 19 million
Spiritism 15 million
Judaism 14 million
Bahá'í Faith 7 million
Jehovah's Witnesses 6.5 million
Jainism 4.2 million
Shinto 4 million (see below)
Cao Dai 4 million
Zoroastrianism 2.6 million
Tenrikyo 2 million
Neo-Paganism 1 million
Unitarian Universalism 800,000
Rastafari movement 600,000
Christianity encompasses many different denominations but the statistics in the source for this document consider most of them all together for the purposes of analysis (except Unitarians and Rastafarians). The detailed country-by-country figures given by the primary source for this section[20] sum to a range lower than the 2.1 Billion total cited in the summary "Major Religions of the World" list[21] (itself derived from the World Christian Encyclopedia[22]).
The high end estimate for Islam from the source for the table above is 1.4 billion:
Islam: Contemporary figures for Islam are usually between 900 million and 1.4 billion, with 1 billion being a figure frequently given in comparative religion texts, probably because it's such a nice, round number[23].
The high end estimate for Hinduism from the source for the table above is 1.4 billion:
Hinduism: The highest figure we've seen for Hinduism (1.4 billion, Clarke, Peter B., editor), The Religions of the World: Understanding the Living Faiths, Marshall Editions Limited: USA (1993); pg. 125.) is actually higher than the highest figure we've seen for Islam. But this is an aberration. World Hinduism adherent figures are usually between 850 million and one billion.[24]
Shinto is a special case due to shrine-reporting versus self-reporting. Since the 17th century, there have been laws in Japan requiring registration with Shinto shrines. Because of this, 75-90% of all Japanese are listed on shrine rolls, greatly inflating the apparent number of adherents. When asked in polls, only about 3.3% of Japanese people identify themselves as "Shinto."[25] However, many who do not consider themselves "Shintoists" still practice Shinto rituals.
In ranking religious denominations, the Roman Catholic Church is the largest single denomination within Christianity, Sunni Islam within Islam, and Vaishnavism within Hinduism. It is difficult to say whether there are more Roman Catholics or Sunnis, as the numbers are roughly equal, and exact counts are impossible, because some members though legally accepted in those denominations may have renounced their faith or have converted quietly