Wikipedia says Chehlum Congregation in Karbala Largest Gathering in the World

Wikipedia, the free online encyclopedia, has described the assembly of Shia Muslims in Karbala on Arbaeen as the largest pilgrimage gathering on Earth.

Arbaeen marks the anniversary of the 40th day after the battle of Karbala on Ashura in which Imam Hussein (AS) and 72 of his companions were martyred.
This year, some 19 million Shia Muslims gathered in the holy city of Karbala on Arbaeen (Thursday, January 3), the encyclopedia wrote, noting that the number of pilgrims last year amounted to 15 million.
It also said that the number was between 10 to 14 million pilgrims in the years 2009 and 2010.

Many people from cities of Iraq go to Karbala on foot to be in the city on the day of Arbaeen as a gesture of sympathy with the batch of the Ahl-ul-Bayt (AS) prisoners who were forced by the army of Yazid to go on foot to Karbala from Levant (Syria) after the event of Ashura.
Many foreign Shia Muslims have also joined the Iraqi pilgrims every year since the fall of Saddam Hussein.
Karbala Friday Prayer Imam: More Than 17 Million Pilgrims Mark Arbaeen in Karbala
On the other hand, His eminence Sayed Ahmed EL-Safy, stated some of the statistics concerning the pilgrimage. He declared the number of pilgrims between 16 to 17 million pilgrims. 750000 pilgrims were from the Arab and foreign countries.
Friday's prayer, second sermon, 4-Jan-2013, the representative of the supreme religious authority and the Friday sermon preacher in Karbala city; His Eminence Sayed Ahmed EL-Safy, delivered the greeting and the good deed prayer of the supreme religious authority in Najaf to the pilgrims of Imam Hussein (p.b.u.h).
He expressed his thankfulness to the central and local governments, land and sea border crossing points that contributed actively by facilitating obstacles for pilgrims, he also appreciated the efforts for the directors of the Husseinian processions and leagues and the personnel of the two holy shrines that performed a great attitude and faithful extraordinary efforts in the Fortieth pilgrimage, as well as the efforts performed by the volunteers from different cities of Iraq emphasizing the great role that the vehicles departments of the two holy shrines and the supporting vehicles of different directorates and ministries.
His eminence praised the pilgrims who were the best example of adhering to the jurisprudential rules, performing prayers at the exact time, and being aware of keeping the public properties.
The second focus of the sermon, his Eminence also stated some of the statistics concerning the pilgrimage. He declared the number of pilgrims between 16 to 17 million pilgrims. 750000 pilgrims were from the Arab and foreign countries, as well as he mentioned that the processions were 17500, 6150 were within the managerial limits of Karbala city.
The third focus, his Eminence mentioned the aggravated political status that Iraq going through nowadays; calling the political powers in the parliament and the government for making plans dealing with those crises radically. He said further that escalating by any means would cause nothing but Disastrous consequences. By the public interest; he focused on the necessity of taking care of the public interest more than the factious interest.
His Eminence concluded with calling the debating political parties for avoiding the provocative statements or convulsive terms that would lead to scattering the national unity.