How about two doubles or three singles

Normally you'd do sconces beside the mirrors, not above. Are you, instead, using these sconces like light bars/vanity lights? Is your electrician not able to move the junction boxes? You know, when you have a larger back plate like these do, you have a bit of space to play with your placement.

Any more than two is overkill in the vanity space. Move the boxes and center one double fixture over each sink, through wall to wall mirror. I'd stick with something smaller as well! but still a double. Three finishes below, and less clunky heavy, with better shading to eliminate glare. With two, you'll have 240 watts of light over the vanity. PLENTY!!

https://www.restorationhardware.com/catalog/product/product.jsp?productId=prod5210042&categoryId=cat2120031

Thanks for the advice. These sconces are at face height - 66". I am not a fan of overhead lights in front of my mirror. but maybe i need to rethink that.

@anobed

That is too, too low for your light style. Go up to at least 84 inches above the floor and center something like the link in my first post over each of the sinks Yours are literally going to be IN your face, and they are all wrong in style to go alongside face height at that 66 inch height. The length of your vanity and position of your sinks really do not lend themselves to more than two small double or three VERY light and delicate single sconces of 60 watts each. But lighter and SHADED, as the style will be a glare in your eye with your clear glass single at 66 inches.

My point is yours fall in a different category than decorative "sconce" though they may be called "sconce" : ) A bit heavy for 75 inches and they look like they belong ABOVE.

If you want three as you are wired ....at 66 inches? I'd look for something like this (four finishes).....and not have to squint from glare. Put them THROUGH the mirror. You can always augment with recessed in the ceiling. Any more than that will look like clutter in 75 inches of vanity

https://www.restorationhardware.com/catalog/product/product.jsp?productId=prod5210042&categoryId=cat2120031

You can also do them on each of the walls and maybe one in middle

Better yet get long arm ones and put on the side walls with nothing in middle

There is only one side wall in the plan so that last one might not work. I like Mistry's other suggestions.

There's no side wall by the opening?