The working concentration of ampicillin is 100 ug/mL. It's typical to have your stock at 100 mg/mL (x1000) and then for example, to 100 mL LB add 100 uL of stock ampicillin, thereby resulting in a final concentration of 100 ug/mL.

If your LB was 250 mL, you'd need to add 250 uL of your 1000x stock.