Each set is different from what I've seen. In some cases, if the letters were SN35 for each, and a letter was in the word or phrase twice, it would be SN70. Three times, then SN105. In other cases, one can find two copies of the same letter with the same serial number (e.g., two letter O's with 23/35). Some sets serialize the whole run for the one player (as in, if the last name to be spelled was "WOOD", and 25 copies of each, the cards would be SN100). Then they have the "W" with SN1/100, 5/100, 9/100, etc. The first "O" would be 2/100, 6/100, etc., Second "O" would be 3/100, 7/100, etc., and the "D" would be 4/100, 8/100, etc. So the only way you'd know "which O" you had was from the first number in the SN. There may have been other numbering scenarios for these, but those are the 3 types that I can remember seeing and dealing with.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).