I would put them a little less than 12 feet apart. Here is the reasoning- you will need to put a 2X4 at the top along the top of your 4X4 posts, so that you can screw your wire into it so it wont sag and won't be too easy for predators to pull out. The 2X4's come in 12 foot lengths. (or you could do 8 foot lengths, and space your posts at 8 ft apart- but you'd get more for your money out of the 12 ft length) So if your posts are inside of 12 feet, then you can butt up your 2X4's (top and bottom) without having to cut. Just lay it all out and measure it all out.