How much more butter should I add to my chocolate chip cookes to keep them soft?

I wouldn't add more butter. I think that is actually likely to make them spread more as they bake and that might make them more crispy. I would add a little more flour, a few tablespoons of it, and a little liquid such as water or milk, only a couple of tablespoons. The extra flour makes a stiffer dough which will not spread. Cookies that spread more tend to lose more moisture as they bake. Use larger amounts of dough for each cookie. Thicker cookies stay soft longer. You could also try using all brown sugar, because it has more moisture than white sugar. Use either all brown sugar or the water; I wouldn't do both at once for the first try at modifying the recipe. Raise the baking temperature to 400F and don't bake too long. Try no more than 8 minutes, making sure to remove them from the oven when the edges are slightly browned and the middle is still pale.

My suggestion is to use vegetable shortening rather than butter. Trust me, all your cookies will improve!