how to find the no.of moles in a particular ionic compound,for ex:KCl is 2,K3PO4 is 4 ,how?

First determine what ions you have and how many of each. 

In the given example,

For KCl, answer is 2.

Explanation

you have K⁺ potassium ions and Cl^- chloride ions. 
Now how many are there?

There are 1 K⁺ ions and 1 Cl^- ion for a total of 2 ions in the formula. Therefore the answer is 2.

Similarly,

For K₃PO₄, anser is4.

Explanation

you have K⁺ potassium ions and PO₄^- phosphate ions. 

Now how many are there?

There are 3 K⁺ ions and 1 PO₄^- ions for a total of 4 ions in the formula. Therefore the answer is 4.