DIY & Home Improvement

At least 1 on each floor, including basement and attic.
1 inside each bedroom - and the bedrooms ones do not count towards the "1 on each floor" rule.
1 in the room at the top of any stairs
In multifamily houses, you also have to put one on each level of any stairs.

And in a new house, they'd be wired together so that if one goes off, it sets off the others.

As for fire dangers, generally speaking the stuff permanently attached to the house is built with some safety features to prevent a fire. Of those devices, cooking devices are the most common cause, followed by dryers. Fireplaces are usually not a cause. They are generally rarely used, and even when they are, they're designed with fire in mind.

Most house fires come from non-permanently-installed stuff, like cigarettes, candles, space heaters, hot plates, and so on.

Smoke detectors are generally placed away from the kitchen unless there's no better place for them. If they're near the kitchen you'll want one of the "photo" smoke detectors, because they are less sensitive and thus less likely to false alarm when you burn dinner.