How many electrons can be described by the quantum numbers n = 3, l = 3, ml = 1?

There are a total of four quantum numbers that govern the electrons. These are the principal quantum number (n), the angular quantum number (l), the magnetic quantum number (ml) and lastly the spin quantum number (ms). Each electron has a unit set of the four quantum numbers.

Since the first 3 quantum numbers is already specified, so the remaining to be filled is the ms quantum number which can only have values of +1/2 and -1/2. So we have 2 electrons.

2 electrons

atomic number of carbon is 6

electronic configuration of carbon is 1s²2s²2p²

orbital diagram of c is as shown here in the image.