For youth it's 35 feet for older its 40 feet
Actually to reduce cavities you need to brush you teeth at least 3 times a day.
But if you don't want cavities, plaque, or enamel on your teeth you should buy a very effective toothbrush and mouthwash to kep your teeth nice and clean.
You don't want Teeth with all kind of cavities in it, right? You want shiny and clean teeth. And to answer you question it all depends on what toothpaste you buy. Sensodyne Pronamel Gentle Whitening Toothpaste.
That toothpaste is designed for sensitive teeth and whitens your teeth. But it all depends on the person who buys the toothpaste and which one they like.
The type of toothpaste I use is Ultrabrite with advanced whitening. But as I said before, it is what the person likes...
I hope this answer helped you! :)