There are different stats about this. It is mentioned in different references, numbers ranging from 4000 to even 120,000.
What is correct casualty count?
Also did this attack really happened in history?
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The attack of Wahhabi Muslims on Karbala did happen in history, during the long quest of Wahhabi rise to power in creation of Saudi Arabia. It's just a few years before Wahhabis claimed the city of Mecca and Medina (1803). It was sacked because its role as one of the major center of Shi'i learning; the religious shrines and pilgrimage caravans were destroyed. One of the most notable tomb, the tomb of Husayn bin Ali (the grandson of the Prophet) was also destroyed.
Several authors hold different dates on when the events happened. Some wrote it happened on 1802, however most noted it happened on 1801. Here are some sources I've found noting the incident:
I haven't been able to find the number of casualties though. Perhaps @MarkCWallace could help on this matter.