When Tyler and Clay are shooting, the police show up just as Clay reaches the end of his magazine as evidenced by the slide going fully back. Though he and Tyler flee the scene, in the next segment with Clay has the gun and the slide is back in it's normal position. He's never been shown how to load the gun and it's not suggested that he stopped to buy more ammo.
The time placement (April 18-20, 2018) is inaccurate. The 20th is a Saturday, although in reality, the 20th is a Friday.