If you look at a battery, it has two terminals: one positive end (+) (called the cathode) and one negative end (-) (called the anode). These two ends are made of two different metals and inside the battery a chemical reaction removes electrons from the positive end and adds electrons to the negative end. Adding a wire connecting the (+) and (-) ends closes the circuit and provides a path for the electrons at the negative end to flow through the wire to the positive end creating an electric current in the wire. A stronger battery will have a bigger difference in electrons between the (+) and (-) ends than a weaker battery.

A very good summary of making a coin battery can be found at Khan Academy:
penny battery

The smallest battery that could be made with coins would have only two coins: one for the positive end and one for the negative end. You can do this by sanding down one side of a penny to expose the zinc metal and then creating a sandwich of two pennies with a salt bridge between them. The salt bridge can be made by soaking a piece of mat-board in a salt vinegar solution. This would be a single electrochemical cell. Most batteries are made by stacking several electrochemical cells together. Each time you add another layer of salt and another coin on top, you are adding another cell to your battery. A typical LED light requires 1.5 Volts so you could light an LED light with 3-4 cells (4-5 coins).

To light a small light bulb like one inside a flashlight would require many more coins because these light bulbs require more current. A good experiment would be to hook up a voltmeter to a coin battery, measure the volts, and record in a table how the voltage increases as you add more coins. By recording this data you could make predictions as to how many coins would be required to achieve a certain voltage.

That depends on how the battery is going to work. If you're just using the coins for the metal, then one is enough (more coins means more voltage, and more power, and longer lifetime, but one is all you need).

That's a fun question! Here's a good answer I found by pasting your question into a google search:

here

Light an LED with five cents. Use 5 pennies newer than 1982 [the metals in pennies changed in 1982]. You need to sand the copper off 1 side of 4 pennies so there is zinc on 1 side. LED light bulbs don't need much energy to light. The directions are at the website. I chose the Exploratorium website, because I know they do good science for kids.

Here are some other links:

2. link 2 - this YouTube video shows how hard it is to sand the copper off the pennies and says you can use silver-colored zinc washers from the hardware store so you don't have to sand the copper off the pennies.

3. link 3 - this site uses 4 pennies and 4 nickels, so you don't have to sand the copper off 1 side of the pennies. But you use a volt meter to measure the voltage from the coin-battery.

4. link 4 - this site uses pennies and nickels, too.

Thank you for your question, Lily - maybe I'll try this with my granddaughters!