You can get 3x 16 Guineas = 48 in total.
To get the Cossack you have to buy some additional Guineas in the premium shop (for example best offer: 0,89€ for ten Guineas - limited to one package per account - you have to be logged in to see the offer). The amount of additional Guineas depends on your efforts in the RN Arc. But in the best scenario you could get a new tier8 Premium ship for less then 1€. That is a huge bargain!