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They chill the fuel and oxidizer to slush, just above their freezing point, to fit more of it into the same volume and save on structure.

How many Falcon 9s have been launched using this fueling method?

As far as I know that was one of the defining changes of the Falcon 9 Full-Thrust, which flew first in December 2015 with Orbcomm OG2 M2. So, every Falcon 9 this year, plus one.

Thank you

At least the last two launches, off the top of my head.

Liquid O2 aka oxygen is a common and cryogenic oxidizer. Loading cold liquid fuels also has benefits like lower volume, but few things stay liquid at cryogenic temperatures.

PS: When dealing with rockets oxidizer is also a fuel because they need both it and something to burn.

For oxygen to be liquid it needs high pressure and low temperature.

Actually, atmospheric pressure is enough, and oxygen boils at a slightly higher temperature than nitrogen.

thanks, I should have said "or."