How can we add list or multiple values for a single key in web.config in asp .net

Add in web config as comma separated valued like

<add key = "xyz" value="val1, val2, val3"/>

access them as

  string[]  xyzValues = System.Configuration.ConfigurationManager.AppSettings["xyz"].Split(",");

You can't do it directly but with a little more coding you may have custom config section in your config files. https://msdn.microsoft.com/en-us/library/2tw134k3.aspx link describes how to do custom config.

Other way could be use a separator in value and define multiple values and then use that values using split function(like other have mentioned)