Killing them was not her goal.
Daenarys didn't want to just blindly kill as many soldiers as possible. She burnt the supply wagons just to show the Lannister and Tarly armies what she could do. If her dragon can burn wagons, the dragon can just as easily burn men. She did burn some on the front line, just enough to let the Dothraki to break the line.
I know what Cersei has told you. That I have come to destroy your cites, burn down your homes, murder you, and orphan your children. I am not here to murder and all that I want to destroy is the wheel that has rolled over rich and poor to the benefit of no one but the Cersei Lannister's of the world. - Daenarys Targaryen in episode 5 of season 7
Converting former enemies into her soldiers was her goal.
The first thing she does after capturing them is give them a choice. She tells them that Cersei's words about her are just propaganda. After she capture them, she burnt only a few that refused to bend the knee. The rest she accepted into her own army.
She wanted them to live so she could give them a choice to join her. Either join her against Cersei, and also in the fight against the army of the dead.
I offer you a choice. Bend the knee and join me - together we will live the world a better place than we found it - or refuse and die. - Daenarys Targaryen in episode 5 of season 7