Since there is only one world, the use of a plural s can only make it less clear whether it is grammatical or not. An improvement might be to add in championship - World Championship Competitors, or World Championships Competitors. Or to remove the world - Competitors.
A single letter X in mathematical notation indicates an unknown. You would write the plurals as Xs or perhaps as Xs (the former has both X and s in bold; the latter has only the X in bold.)
Note how Xs and Ys are used at our sister mathematics site.