2p orbital has 3 orbitals that is px,py and pz hence in 3 orbitals there can be maximum of 6 electrons in each orbitals there will be one +1/2 spin and other -1/2.therefore there are 6/2=3 -1/2 spin orbit
In p there are 3 orbitals each having the capacity to accommodate 2 electrons...... In one orbital only 1 electron can have one type of spin quantum number i.e.+ve or -ve