When a solution is diluted, the number of moles of the solute decreases. what would happen to the molarity?
The number of moles of solute does not decrease on dilution.
For example suppose you add 2 moles of NaOH to 100cm3.
Now if you dilute this solution to 1000cm3 you  are still going to have 2 moles of NaOH.
But now the total volume has changed ,hence the concentration also changes.