It comes from respiration. Once the ATP is broken down, the ADP and P make their way back to the mitochondria. There, they are once again linked together to make ATP during the steps of cellular respiration.
Remember, cellular respiration does not make ATP from scratch. It simply takes ADP and P which are already there and links them up again. Thus, a cell needs only so much ADP and P. After that, the ADP and P are fused to make ATP, the ATP is then broken down to release energy where it is needed, and the resulting ADP and P are shipped back to the mitochondria to start the process all over again.