A few random thoughts:
 As has been mentioned, it's not so much how many rounds you shoot, it's how well you shot them. Pay attention to what you're doing and make every shot good.
 As someone else mentioned, only perfect practice make perfect. And practice makes permanent. So if you keep doing something badly over and over, you will not get better at it. You will only become expert at doing it badly.
 Shorter but more frequent practice is better than longer, less frequent practice. If you keep practicing after you've started to get fatigued or your interest and concentration has started to flag, your practice can start to get sloppy and undo the good you did earlier in the day. Frequent breaks during practice can also help.
 It can also be helpful to understand the way humans learn a physical skill. In learning a physical skill, we all go through a four step process:
- unconscious incompetence, we can't do something and we don't even know how to do it;
- conscious incompetence, we can't physically do something, at least consistently, even though we know in our mind how to do it;
- conscious competence, we know how to do something and can do it properly consistently, but only if we think about what we're doing and concentrate on doing it properly; and
- unconscious competence, at this final stage we know how to do something and can do it reflexively, on demand and without having to think about it.
To get to the third stage, you need to think through the physical task consciously in order to do it perfectly. Then going from conscious competence to unconscious competence is usually thought to take around 5,000 good repetitions. The good news is that, in the case of shooting, dry practice will count. The bad news is that poor repetitions don't count and can set you back.
If one has reached the stage of unconscious competence he will still need to practice regularly and properly to maintain proficiency, but it's easier to maintain it once achieved than it was to first achieve it.