How many electrons does phosphorus have in its valence shell and neutrons in its nucleus, respectively?

If you look at the periodic table of elements, you can see that atomic number for phosphorus is 15. It means that it has 15 electrons and 15 protons total. Now you can write configuration for P which is:  1s22s22p63s23p3. or [Ne] 3s2 3p3 
From here, you can see that it has 5 valence electrons (s2+p3).

In the periodic table of elements the number of protons+ number of neutrons is determined as atomic mass. Atomic mass of the P is 30. 
number of neutrons = atomic mass-atomic number 
number of neutrons = 30-15
number of neutrons= 15 

Abbreviated as mg-25.