Pretty sure this is Reboot (2013) by Amy Tintera, the first book in what is currently a duology.
Five years ago, Wren Connolly was shot three times in the chest. After 178 minutes she came back as a Reboot: stronger, faster, able to heal, and less emotional. The longer Reboots are dead, the less human they are when they return. Wren 178 is the deadliest Reboot in the Republic of Texas. Now seventeen years old, she serves as a soldier for HARC (Human Advancement and Repopulation Corporation).
Wren’s favorite part of the job is training new Reboots, but her latest newbie is the worst she’s ever seen. As a 22, Callum Reyes is practically human. His reflexes are too slow, he’s always asking questions, and his ever-present smile is freaking her out. Yet there’s something about him she can’t ignore. When Callum refuses to follow an order, Wren is given one last chance to get him in line—or she’ll have to eliminate him. Wren has never disobeyed before and knows if she does, she’ll be eliminated, too. But she has also never felt as alive as she does around Callum.
The perfect soldier is done taking orders.
This user review from Goodreads adds a bit more detail on the loss of humanity aspect:
Set in a dismal future where the dead can be brought back to life and enslaved by humans, these "rebooted" individuals are given a number which is how many minutes they were dead before being brought back - the higher that number, the less humanity still lingers inside them. Wren 178 is the highest number and the deadliest Reboot in the Republic of Texas, she is cold, emotionless and completely focused on following orders and completing the task at hand. That is... until a newbie with one of the lowest numbers she has ever seen arrives. Callum 22 was dead for such a short amount of time that he is almost human and, for the first time, he encourages Wren to challenge her beliefs, her worth and if she really wants to continue being a human slave forever.
And this excerpt from the book itself confirms the part about the Reboots sitting at different tables in the cafeteria, based on how many minutes they were dead for before being brought back:
The cafeteria was one floor down, through a pair of big red doors that warned of the dangers inside. We stepped into the room, which was blindingly white except for the clear glass that lined the upper portion of one wall. HARC officers were stationed on the other side, behind the guns mounted to the glass.
Most of the Reboots were already there, hundreds of them sitting on little round plastic seats at long tables. The rows of bright eyes shining out against pale skin looked like a string of lights down every table. The smell of death hung in the air, causing most humans who entered to wrinkle their noses. I rarely noticed anymore.
Ever and I didn't eat together. Once we got our food, she split off to the table for the Under-sixties with her tray and I sat down at the table for One-twenties and higher. The only one who came close to my number was Hugo, at One-fifty.
Marie One-thirty nodded at me as I sat down, as did a few others, but Reboots over 120 minutes were not known for their social skills. There was rarely much talking. The rest of the room was noisy, though; the chatter of Reboots filled the cafeteria.