If the total mass does not change before and after a chemical reaction then why Ca+H2O=>CaOH+H2 has 2 hydrogen atoms at first then 3 after?

Your equation is not balanced:
This is the correctly balanced equation:
Ca + 2H2O → Ca(OH)2 + H2
Now you will see that there is identical atoms and mass on both sides of the equation.

Your reaction equation is NOT Balanced.
The Balanced reaction equation is: -

Ca(s) + 2H2O(l) = Ca(OH)2(aq) + H2(g)

You will notice that on either side there is: -
1 x Ca
4 x H
2 x O

Remember the Law of Conservation of Mass. The total mass of reactants must equal to the total mass of the products. Hence it follows that the number and type of atoms must also balance.