If you break or severly sprain your ankle, and you get a boot, do you need crutches?

You need to do what it takes to take care of yourself and if that means wearing boots and using crutches, go for it. There are no rules that state you can't use both. And if anyone asks why you need both, either ignore them or tell them MYOB. Boots are probably a good idea and will keep the swelling down and give that ankle support. The crutches will ensure your success at not using your ankle more than you should and keep you from falling and/or reinjuring it.