How much volume of gases could be added to Mars atmosphere that the planet gravity can retain? This article Mars atmosphere states that


  Today Mars atmosphere is very thin, but it was not always like this. Around 3.5 billion years ago, the red planet had a thick enough atmosphere for liquid water to run smoothly on its surface


Suppose you could increase Mars atmosphere volume by melting frozen CO2 or whatever. How much volume of gases could be added to Mars atmosphere that the planet gravity can retain?
There are hard limits on what types of gas that Mars can retain based on its temperature and mass 
(Graph of what gasses an astronomical body can retain).

Volume wise, it's not clear. Currently Mars is still losing its atmosphere, so it can't even retain that amount of atmosphere. But if you continually added gas to Mars, there isn't an end point where the planet could no longer hold the atmosphere being added. It just increases the pressure of the atmosphere at the surface and only slightly extends the height of the atmosphere. And after it stopped being added, Mars would slowly lose the atmosphere again.