There are currently 13 million Mormons, with another million added about every three years. While the National Council of Churches lists the Mormon church as the fourth fastest growing church in the U.S., the Church itself does not make “statistical comparisons with other churches and makes no claim to be the fastest-growing Christian denomination.” (Some critics mistakenly believe that the Church asserts itself to be the fastest growing church.)

The preoccupation with membership statistics comes by scriptural mandate: “And after they had been received unto baptism … their names were taken, that they might be remembered and nourished by the good word of God” (Moroni 6:4).

Not all Church members are “active,” but the Church does not remove names from its membership records because of inactivity. The baptismal covenant through which persons become Church members is “not [to] be lightly made and is not casually dismissed.” Church growth is fastest in Africa and South America, slower in Northern Europe.

The faithfulness of members is more important than statistics:

“Ultimately, the strength of the Church is really measured by the devotion and commitment of its members,” said Elder M. Russell Ballard of the Quorum of the Twelve Apostles. “The Lord has never given us a mandate to be the biggest Church — in fact, He has said our numbers will be comparatively few — but He has asked that we commit ourselves to living and sharing the gospel of Jesus Christ.”

Source: Church Growth Statistics