Question

How many electrons does a co atom have in its 3d subshell?

Answer 1

Co is the symbol for element cobalt.  Atomic number of Co is 27.
1st shell has s orbital and can hold up to 2 electrons.
2nd shell has s and p orbital and can hold upto 8 electrons 
3rd shell has s, p and d orbital and can hold upto 18 electrons 
In Co electronic configuration upto p orbital in 3rd energy shell is
1s² 2s² 2p⁶ 3s² 3p⁶
After 3p orbital, electrons start filling in the s orbital in 4th energy shell as 4s orbital has lower energy than 3d therefore it fills 4s before 3d, after it fills 2 electrons in 4s, remaining electrons fill 3d orbital 

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁷
in the 3d subshell there are 7 electrons