one over each mirror, for sure.

One over each sink like maybe 3 bulbs each for adequate lighting without shadows. Bright lights on a dimmer switch is ideal.

Does it matter that it is only one large mirror versus individual mirrors over each sink? We many times have 3 teens getting ready at the mirror at the same time so need the larger mirror if possible.

If you don't spread the light across the mirror you will get shadows which make shaving or applying make-up more difficult. This is what we did in our hall bath (this is a 72" vanity):

It doesn’t matter that the mirror spans entire area. Important to get adequate lighting for the area. I like Kandrewspa’s photo you might consider.

A lighted mirror full span would be awesome for makeup with additional lighting in the ceiling.

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We might recommend doing a light over each sink, even though you have one mirror. Below are two bathroom designs of ours. Please notice the lighting:

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Two sinks, two lights….you want light on the face and hair. Use a hand mirror to see the back of the hair.