The phrase [あといくつ心臓があれば](javascript:void(0)) translates to "How many more hearts would there need to be" or "If there were how many more hearts." This line is poetic and metaphorical, commonly seen in lyrics or expressive texts. It suggests a longing or need for more capacity to feel or love, symbolized by having multiple hearts. Specifically, this phrase is well known as part of the lyrics from the song "うるうびと" by RADWIMPS. In the song, it expresses a deep emotional yearning, questioning how many hearts would be needed to hold or convey all the feelings the singer has for their loved one. It is not a literal question about actual hearts but a figurative expression of intensity of emotion or love.

Therefore, the meaning of the phrase is to ask metaphorically how many hearts one would need to fully express or contain a strong feeling, usually love or emotional capacity.