Please answer a question about the following article about Oklahoma:

Oklahoma is host to a diverse range of sectors including aviation, energy, transportation equipment, food processing, electronics, and telecommunications. Oklahoma is an important producer of natural gas, aircraft, and food. The state ranks third in the nation for production of natural gas, is the 27th-most agriculturally productive state, and also ranks 5th in production of wheat. Four Fortune 500 companies and six Fortune 1000 companies are headquartered in Oklahoma, and it has been rated one of the most business-friendly states in the nation, with the 7th-lowest tax burden in 2007.

How many Fortune 1000 companies are based in Oklahoma?
six