i think there is no relation between stages and form of ring oscillator. maybe diff oscillator is more common in circuit design.everyone know frequency of vco decrease with increase of stages.ring oscillator stages? it's up to your need!
For oscillator is only important positive loop phase. With single ended you can make it only by taking odd numbers of inverters. In the case of differential, you have possibility to reverse inputs of next stage or not which means that you can easily make positive phase with even or odd numbers.