直接看研究
https://pubmed.ncbi.nlm.nih.gov/9777833/
「The probability of conception increased with increasing sperm concentration up to 40x10(6)/mL, but any higher sperm density was not associated with additional likelihood of pregnancy. 」
也就是每毫升40x10^6以上不會增加受孕成功率
4千萬跟12億兩者對受孕成功率沒有統計上的差異