Ca + 2h2o → ca(oh)2 + h2 how many different elements are involved in the chemical reaction shown?

In the given chemical reaction, Ca + 2h2o → ca(oh)2 + h2, we can see that calcium and water reacts to form calcium hydroxide and and hydrogen. If you want to point out each element in the equation, you can see that we have Ca which is Calcium, H2O which is water made up of the elements Hydrogen and Oxygen, and the rest of the compounds also include these elements.  

So, the different elements include Calcium, hydrogen, and oxygen. I hope I was able to help you with your question. Let me know if you have further questionings based on the information. :)