Assuming you have fresh eggs*, the number of eggs you can make at a time depends on the size and shape of your electric kettle. I have a Chef's Choice Cordless Electric Kettle Model 677 and I can fit 4 eggs comfortably. You need to leave room inside the kettle so that they don't touch the heating element. Once you place the eggs inside fill them so that there is about an inch of water above them.
* If you are not sure your when your eggs were purchased you can test them by dropping them into a glass of cold water. If they sink like a stone then they are great. If they drop and bounce a couple of times they are older but still are okay. If they float they they are bad and should be tossed! I found