Iyas bin Salamah narrated: My father narrated a hadith, he said: A man sneezed in the presence of the Prophet Muhammad (peace be upon him). The Prophet (peace be upon him) said to him, "May Allah have mercy on you." When he sneezed again, the Prophet (peace be upon him) said: "The man has a cold."
Hadith Referenceسنن دارمي / من كتاب الاستئذان / 2696
In this hadith, it is stated that if someone sneezes more than once, it is not obligatory to respond. In Ibn Majah, it is mentioned that the response should be given three times; if the person sneezes more than that, saying "yarhamuk Allah" is not obligatory. Rather, the one who sneezes repeatedly is considered to have a cold (muzkoom), meaning that responding after more than one sneeze is recommended (mustahabb), not obligatory (wajib).