10
Phantom weapons work for all elements, but you can only use 1.
The stat boost to same-type weapons is apparently 30% attack 45% hp, the wiki doesn't have numbers for most of the second skills but apparently they are amazing. On the page with the sword all the comments are talking about how high the combo rate is... this is the same wiki where people sigh every time a weapon gets Rush/Barrage so it must be really impressive.