The amount of ito depends on the type of maki you are doing. Some styles take more, others less. Obviously, the handle size will make the biggest difference. A big fat Odachi handle that is 1X1.5" will take a lot more per inch. A kiridashi ... a lot less.
If you look at the math - if a handle has an average diameter* of 1" ( say, 1.25X.75"), it takes about 3.2" of ito per wrap. If using 10mm ito and the handle is 9" long, that is about 7 feet, the twists and scrunching the wraps tight together adds some, and you need a foot or so extra on each end to do the final knot. So if you use the standard guestimate formula, 9+3=12 feet, that is going to be just right. No one likes wasting materials or money, but when doing tsukamaki, coming up a little short would really ruin your day.
*If you are a purist, the formula for calculating the circumference of an ellipse is: Pi * Long axis/2 squared * Short axis/2 squared = Circumference
Additional:
The wrapping stand is an important part of doing a good wrap. While it is possible to do a wrap freehand in your lap, it is not advisable. A firm stand that you can pull down on the ito while wrapping is what you want. You can use any of several types, but the key is being able to continuously rotate the handle and being resistant to side force when pulling the wraps tight. I like the type that works roughly like a knife vise. Mine is made from PVC piping and a simple wood frame. Modified spring clamps are used to hold previous wraps snug when rotating.
I like to make a work mandrel from a bar of mild steel with one end ground to match the nakago so it fits snug in the tsuka. I use it to wrap the handle and then transfer the handle to the blade for installation of the mekugi. This isn't always possible, but when it is it avoids scratching up a finished blade.
