General idea is that a person should offer nafil salaat only after he is caught up on all missed fard qadha salaat. if qadha salaat are still left then it is useless to do nafil salaat. salaat tawbah is just 2 rakaats and then make tawbah for sins. since it is nafil you can add many intentions along with it, salaat tawbah salaat shukrana salaat haajat all in just the 2 rakaats.