Answer:
As much as you can swallow is fine. It is all safe and studies suggest that swallowing semen helps reduce a woman's risk of breast cancer.
Any quantity is fine if the donors are healthy and free of STD's. I've not heard of it preventing breast cancer, but it has vitamins and minerals that would be good for you.
Cancer prevention: Studies suggest that seminal plasma both prevents and fights cancer, particularly breast cancer, reducing risk by "not less than 50 percent." This effect is attributed to its glyco-protein and selenium content, with apoptosis being induced by TGF-Beta
It has been hypothesized that substances in semen condition a mother's immune system to accept the "foreign" proteins found in sperm as well as the resulting fetus and placenta, keeping blood pressure low and thereby reducing the risk of preeclampsia. A study shows that oral sex and swallowing sperm may help make a woman's pregnancy safer and more successful, because she is absorbing her partner's antigens