Current code expects hardwired smoke alarms with battery backup, installed in every sleeping room, outside each separate sleeping area, and on every level of the home including basements. In most North Texas houses, that is more devices than are actually mounted on the ceiling right now.
The requirement people miss most often is interconnection. When alarms are interconnected, triggering one sounds all of them. That difference matters most in the scenario the alarms exist for: a fire starting in a garage or a back room at two in the morning while everyone is asleep at the other end of the house. A single alarm sounding two rooms away is frequently not enough to wake someone.
The second thing people miss is expiration. Smoke alarms have a service life, usually ten years from the date printed on the back of the unit, after which the sensing chamber degrades and false negatives become more likely. If you have never checked that date, it is worth doing this week. Units older than a decade should be replaced regardless of whether they still chirp when tested.
Carbon monoxide is a separate question with overlapping answers. Homes with fuel-burning appliances or an attached garage need CO detection as well, and combination smoke and CO units handle both from one device on one circuit. Placement rules differ slightly from smoke alarms, which is worth getting right rather than assuming.
Hardwiring and interconnecting a whole house is generally a half-day job in an accessible home, and less if there is existing interconnect wiring to reuse. It is one of the least expensive things you can do to a house that meaningfully changes an outcome.