I had this problem a few years ago, and discovered when I went to replace my keyboard (I had drowned it) that the letters are only printed on the keys now, whereas they used to be incorporated into the original mould. Mine was a white board and now only Mac seems to make them in white. So I have had to go over to black boards that wear off after a few months.
I bought a pack of stick-on numbers from Amazon. They have lasted very well, you can hardly tell they are there. They are only made of paper and cost around £3, but it's better than always buying new keyboards. I tried white marker pen, and silver, it wore off before the day was over.