If you are a good or better applicant and you're not strictly applying to "stretch" programs, the 10 you get for your ERAS fee should be enough. if you have geographic restrictions, couples match issues or any red flags on your app, apply to more. Honestly, applying is cheap, casting a wide net doesn't really cost that much. You'll have (hopefully) the option to trim things down when it comes to interviewing. And you can always apply to more places.
I had a decent Step 1, AOA, HP and Honors in everything and a PhD when I applied 3y ago. I initially applied to 20 programs and added another 8 or so later. I ended up w/ 24 interview offers and went on 18 interviews which was WAY TOO MANY. If I had to do it again, I'd probably apply to 20 or so and go on 10 interviews. Programs interview 6-10 people for every spot they have so, statistically speaking, if you interview 10 places, you should be more than fine.
Just remember that red flags (low board scores, failed courses or Steps, only Pass in your IM rotation, etc) will change these odds and require you to apply/interview more broadly.
Finally, do NOT underestimate the power of the interview. If you get an interview, you're being seriously considered and you can basically only screw things up so don't screw around.