This prevents electrical shorts or glitches that can cause damage. USB doesn't have this limitation. Even so, newer types of USB devices may need additional drivers installed. USB is well suited for plug and play operation. There are no interrupt conflicts or other messy configuration issues involved in using USB devices.
Most computers have two USB ports.
If you need to plug in more than two USB devices you do so by using a USB hub. A hub plugs into your USB port and provides more than one additional plugs. Common hub sizes include four or seven ports. Many USB devices have hubs build right into them. Most keyboards have another USB port in them so you can daisy chain a mouse, joystick, or other USB device to it.