Where does the Ramban say that in reality we deserve more suffering than Iyov received for every sin we do and it is only because of Hashem's incredible mercy that we don't get anywhere near what we deserve?
You might be looking for this from the Ramban's introduction to Job:
ולכן הצדיק הגמור שיארע לו החטא הקל ועבר בזדון על מצות בוראו ית' עכ"פ ראוי ליענש עליו, והנה אם יאבד האל מגופו בעה"ז כל הטובה אשר נהיתה בעולם ויהי' נידון בייסוריו של איוב כל ימיו, טוב לו משיענש בנפשו להיות נידון בעול' הנשמות בייסורי גיהנם
A totally righteous person who happened to intentionally transgress his creator's command is still worthy of punishment. If God took from his body all the good that ever existed and judged him with Job's suffering, it's still better than being punished in the world of souls with the suffering of hell.
(The world of souls is a spiritual existence separate from the resurrection, according to the Ramban in Sha'ar Hagemul.)
He doesn't say that it's only because of God's mercy that we don't get what we deserve. On the contrary, we do get all that we deserve after death. His point is that a totally righteous person, such as Job, shouldn't be punished at all in this world. If he had sinned, his suffering wouldn't be such a problem, since it could be attributed to his sins.