Proper number of salt bytes for Rfc2898DeriveBytes/PBKDF2
Short Answer: 8 bytes are fine

Longer answer: The point of the salt in a password hash is, to prevent the precalculation of a list of hashes, usually done in the form of rainbow tables. If someone still wants to do precalculation, he will have to build one rainbow table for every single possible salt value, thus you are increasing the storage requirements of your adversary.

With a 8 byte salt, the adversary will need to build $2^{64} \approx 10^{19}$ tables. The size of those tables varies depending on the types and length of password you are trying (you can find a nice overview here), but you typically expect something in the 2-3 digit gigabyte range ($10^{11}-10^{12}$ byte). Putting this together, storing rainbow tables for 8 bytes of salt would require something in the range of $10^{30}$ bytes, which is completely impossible.