Guild Wars 2

Can i only use it on one staff? or can i keep using it as long as i have the item in my inventory?

If you apply the skin, it’s one and done. As long as you save the staff you can continue to transmute it, but both items are destroyed in the process, of creating the new item…

is it account bound like if i use it on my guardian, and wanna transmute to my necro, could i do it?

I doubt that Anrothan. It’s just a skin you apply to a weapon which will become soulbound (if it wasn’t already).