Does God forgive the reciter of Surat Al-Infitar when rain falls, by the number of raindrops?

The statement in question has no basis in Islamic law. What is prescribed upon the descent of rain is the supplication transmitted from the Prophet, peace and blessings be upon him: "O Allah, [make it] a beneficial downpour." As for reciting the Quran, it is prescribed at all times and is not specific to the descent of rain, as explained by Sheikh Ibn Uthaymeen.