Many different connectors can have 20 pins. Some examples of these are:
20 pin P1 connector.
ATX version 2.2 allowed for 24-pin P1 connector, which is backward compatible with the 20-pin P1 connector.
A regular ATX motherboard uses a single 20-pin P1 connector with optional 4-pin auxillary power connector
The P1 power connector is the first ATX power supplies and motherboards used a single power connector. It also has 20 pins.
24 pin
24 pin
Enhanced ATX
24 pin connector is required to supply power to the motherboard.
enhancer atx motherboard
There should be a large connector coming from the Power Supply unit. You may have a 20 pin or a 24 pin connector on your motherboard. Most power supplies come with a 20+4 pin connector which allows them to connect to both 20 and 24 pin mother bards.
20- pin connector
P1 connector