Lots! And its very expensive, especially for offshore schemes where the cables need to be buried or laid. Also, turbines are often connected in long "strings" and so the "string" cable is rated for the peak power from the whole string whereas towards the end of the "string" only a fraction of this power flows. You can taper the cable diameter down the string to compensate for this to minimise CAPEX, but that means keeping stocks of different cables, careful protection settings, etc. and this may cause more confusion/difficulty than simply installing a single diameter.
The cost is a serious issue for offshore schemes, being a significant part of the overall scheme cost since the cumulative cable length is long, and the main aim for developers is to keep the overall cost of the scheme down to competetive levels.
So, all of the above sounds very expensive and "bad" use of copper.
However, the voltage of collection is in the 11-33kV range and so cable diameters per MW are suitably reduced. Also, compared to the load factor of (for example) your house, where the copper in the cables is only used towards its peak ratings for very small amounts of time, the use of copper in windfarms is actually quite effective. Consider for example the chunky copper cable in your home connected to the electric shower. Its ~40A capacity to supply about 9.2kW @ 230V. That would be capable of supplying 221kWh if used at its full rating for 24 hours. However, every day it's probably only used for 10-20 minutes (2-4 people?), so only 1.5-3kWh are actually supplied via this cable. Thats a load factor of 0.7-1.4%, which is far less effective use of copper than your average wind farm cable.
The figures are just as bad if you start to look at your domestic 32A rings which supply TVs, table lamps etc. On a typical house with 3x 32A rings and a single 40A shower supply, thats a peak capacity of about 31kW (which you probably never come close to achieving) but the average daily use is only about 12kWh .Thats a load factor of 1.6%, and I didn't include other stuff like lighting circuits and garage wiring which are all included in that average domestic 12kWh consumption statistic.
So, if "inefficient" use of copper is your worry, perhaps start in the home first!
Dr. Andrew Roscoehttp://personal.strath.ac.uk/andrew.j.roscoe