Four digit number, first digit is the last digit of the year they were born then sequentially number until you get to the last calf.  Make each year a different color tag.  Ritchey has eight (I think) different colors so even if you do end up with the same number in two cows they will have different color tags.  This year our tags will be blue and start with number "7001".  With a four digit numerical ID you don't need a full alphabet in order to tattoo your ID number if you choose to do that.  This number scheme will only work as long as you don't have more than 999 cows, I am assuming that is not the case with you but perhaps I am wrong.  Cows will lose their tags, you will need some permanent way to identify them, that is why we tattoo everything at birth with their permanent ID.  Calves get a large tag and those stay with them, unless they tear their ear or the button, until they leave the place.