Before fixing sconces to the mirror, I would tape their outlines on the glass and stand at the sink to check the light at face height. Two fixtures flanking where you actually stand can give more even facial light than one light above, but leave enough clear mirror width and check for any cabinet-door swing. Ask your installer and the fixture and mirror suppliers to confirm that the glass, mounting hardware, wiring route, and bathroom suitability are compatible; drilling or bonding onto a finished mirror is not something to improvise. If the mirror is already installed, wall-mounted sconces beside it or a bar above may be simpler. A photo plus the mirror and sink dimensions would help narrow the placement.

There is no exact rule.

What is the size of the vanity? How many sinks? Spacing between sinks and any side walls? Mirror height? Ceiling height? Size of sconces?

The math all needs to be figured out before anything is ordered/built.