How many wind turbines are needed to make a wind farm?

Humorous answer:just the one, but you'd need to find a lot of celebrities and televangelists to blow hot air on it Technical Answer:

The largest individual wind turbine available today is the Enercon E-126 costing $3.5 million, which can generate about 20 million kilowatt hours per year.

The electricity consumption of the USA is approximately 3,892,000,000,000 kWh/year, therefore, assuming only these turbine were used, and they were sited suitably to receive an appropriate spread of wind over them throughout the year, the number of turbines required would be 194,600. Which would bring the total cost to $681,100,000,000

Answer

The above answer is a great start! There are a few missing elements though. Wind systems are never able to receive wind consistently. Typical wind averages are closer to 5 to 10%. That multiplies the number by ten. Additionally, the grid itself consumes massive amounts of power. Which would more than double the number again. Multiplying the above number by twenty would be a start point. That also assumes that all units would work at all times and that backup fossil fuel plants would be available and idling.