For a 2048-bit modulus, based on current knowledge of attacks: you can use up to $k=3$ primes without any loss in security.  Using $k=4$ primes apparently causes some loss in security (it's not clear to me exactly how much loss it causes, though).

I've found two sources that support this conclusion:


The blog post Multi-prime RSA trade offs analyzes the security of a 2048-bit $k$-prime modulus against the NFS and ECM factoring algorithms.  For $k=2$ and $k=3$, the security level is 107 bits (NFS is the best attack).  For $k=4$, the article claims that the security level is 106 bits (ECM is slightly faster than NFS for four primes), so we've lost about one bit of security, though this estimate seems like it might over-simplify.
Table 3 of the paper Unbelievable Security: Matching AES security using public key systems also addresses this issue.  It suggests that, for a 2048-bit modulus, $k=3$ primes offers no measurable loss of security.  Starting in 2030, $k=4$ primes will offer no loss in security (it changes over time because NFS factoring speeds up faster than ECM factoring).  Here is Table 3:




Multiple-prime RSA; how many primes can I use, for a 2048-bit modulus?
The Talmud (Makkot 23B)and the Zohar (1:170B - unverified English translation here) say that the 248 positive commandments correspond to the 248 limbs of the human body. (The Mishna (Ohalot 1:8) lists the 248 Halachic limbs). The Talmud also says that the 365 Negative commandments correspond to the days of the year.

The Zohar adds that the 365 negative commandments also correspond to the 365 sinews in the human body (See Yonatan Ben Uziel Bereshit 1:27), and connects them to the days of the year.

R' Chaim Vital, in Shaar HaKavanot (Shaar 1, Part 1) says that "Each of the 248 spiritual limbs gets its nourishment from a particular mitzvah that corresponds to that limb. When a person fails to perform that particular mitzvah, the corresponding limb will lack its proper nourishment..." (translation from here)

Is there any source which tells us which limb (and/or sinew) each Mitzvah corresponds to?

As an example, the Zohar referenced above (1:170B), connects Gid Hanasheh (sciatic nerve), one of the 365 sinews, to the prohibition of eating on Tisha B'av.
