If the new water temperature, PH,KH,GH were the same as the tank water there would be no limit as Tim said. That is very rare to be that lucky. What the water changing does is dilute any pollution in the tank from fish waste. If we change some water and it is a little bit different than the tank, the fish will not be bothered much. But if we put a lot of water in that is not the same temperature, it may chill them. We try to change enough water to get the pollution down without shocking the fish. How much that is depends on al;l the things mentioned and is different for most tanks and water. Much depends on the water source you have and how well it matches your tank water.