Depending on the size of the network, one or more APs might be required. Additional APs are used to allow access to more wireless clients and to expand the range of the wireless network. Each AP is limited by a transmissions rangethe distance a client can be from a AP and still get a useable signal. The actual distance depends on the wireless standard being used and the obstructions and environmental conditions between the client and the AP.
Saying that an AP is used to extend a wired LAN to wireless clients doesn't give you the complete picture. A wireless AP today can provide different services in addition to just an access point. Today, the APs might provide many ports that can be used to easily increase the size of the network. Systems can be added and removed from the network with no affect on other systems on the network. Also, many APs provide firewall capabilities and DHCP service. When they are hooked up, they will provide client systems with a private IP address and then prevent Internet traffic from accessing client systems. So in effect, the AP is a switch, a DHCP Server, router, and a firewall.
APs come in all different shapes and sizes. Many are cheaper and designed strictly for home or small office use. Such APs have low powered antennas and limited expansion ports. Higher end APs used for commercial purposes have very high powered antennas enabling them to extend the range that the wireless signal can travel.