How can I bind macros to combinations like letter+number, like X+1, C+1, etc. in World of Warcraft?
You can't do that. The only combinations you can create are with modifier keys like Control, Alt, and Shift.
There are two ways to go about this. Either create a simple macro and drag it to a slot on your action bar that's bound to the combination you want (like Ctrl-1), or modify the macro to do something different if a modifier key is pressed.