You will have to use a separate IdentifyTask for each url. The way to handle this is to loop through all the layers in the map and create a array of mapservice urls and create a IdentifyTask for each add each of those IdentifyTasks to a dojo/promise/all that way you do not work with the results until all your IdentifyTasks are complete. There is several threads about this and if you want actual code you can look at then you can look at my Identify Widget for the Web App Builder:
