From this article at extremetech.com:

"The problem is, today’s Mars bears little resemblance to the primordial planet it must have been. Mars’ atmosphere is a fraction the density of Earth’s — air pressure at the summit of Mount Everest (the highest point on Earth) is 4.89 PSI, while the air pressure at the bottom of Mars’ Hellas Planitia (a 23,465-foot deep crater) is just 0.168 PSI. In other words, the air pressure at the highest point on our planet is 29x higher than the air pressure at the lowest point on Mars."

IOW, Mars' highest atmospheric pressure is almost 30 times lower than the air pressure at the top of Everest. Just think of how much gas you'd need to bring the surface of the entire planet up to just half the air pressure of Everest's peak - about 2.9PSI.

https://www.extremetech.com/extreme/217605-nasa-solves-the-mystery-of-mars-missing-atmosphere#:~:text=Mars'%20atmosphere%20is%20a%20fraction,crater)%20is%20just%200.168%20PSI.