This number, like so many others in the book of Revelation, is symbolic. Seven is the number of fullness. The “thousand” denotes that this full number is large. The wages of sin is always death (Genesis 2:17; Romans 6:23) and so God ensures that the murder of his witnesses is properly avenged.
13 And at that hour there was a great earthquake, and a tenth of the city fell. Seven thousand people were killed in the earthquake, and the rest were terrified and gave glory to the God of heaven.