The power requirements for each graphics card are listed on the side of the package, including the wattage and amperage. The power supply inside your PC should have a label with is output and ratings.
In this example, this 750 G3 power supply has a single +12v rail rated for 62.4A. This power supply will be sufficient for any video card with a requirement of 750w and 62.4A or less. As an example a single RTX 2080Ti is rated for 21A.
You can also use a power supply calculator like the one linked below to find the recommended wattage for your system.
EVGA Power Meter