How can I find out how much coffee is needed to make coffee in an electric percolator?

Try how much you think does in then if its too strong put less in, if its too weak put more in. Then you should find out.

You could do the hit and miss method however you need to know how much water your percolator holds. It should be marked on the inside.

Use 1 1/2 tablespoons of medium/course ground coffee for every 6 ounce cup of water. Most percolators have a clear bubble top and you can see the color of the coffee being brewed. If you prefer milder coffee you can always stop the percolation process as you please by unplugging the appliance.