Is a single "yarhamuk Allah" sufficient for a sneezer if he sneezes more than once, or is it obligatory to say "yarhamuk Allah" for each sneeze?

It is not sufficient to say "Yarhamukallah" (may Allah have mercy on you) to a sneezer a second time to compensate for not saying it the first time. The prescribed practice is to say "Yarhamukallah" to the sneezer up to three times, once for each sneeze. So, if a person sneezes consecutively, the Sunnah is to say it for each sneeze until it reaches three times. It is narrated in a hadith that the Prophet, peace and blessings be upon him, said "Yarhamukallah" to a man who sneezed twice, and on the third time, he said: "The man has a cold." Some scholars have opined that the "Tashmit" (saying "Yarhamukallah") should be done three times, once for each sneeze.