I actually didn't realized this post was in the cards subforum. I'm going into one of the other top 4 competitive subspecialties, so my experiences don't exactly translate but probably would be similar.
I erred on the side of over-applying. I decided I only wanted to go through this process once, and decided I was ok with potentially wasting some money, which I definitely ended up doing. You can add significant expense at both the ERAS application phase and the interview phase.
It's really hard to give advice without knowing the specific circumstances of you and your SO's applications. This is the main questions you should ask yourselves though,
#Is location or prestige more important to you?
Would you guys rather be in a desirable city at less prestigious programs, or more prestige but less desirable city? This will make a big difference. If you are going for prestige over all, then I would guess you should apply to a larger number of programs.
And remember, fellowships generally are more willing to interview you if you have a compelling case for wanting to be in that specific program. I personally know of more than a handful of examples of people emailing/calling PDs after they didn't receive an interview, and ultimately ending up at that program.