The phrase “if able” means an opponent that has fewer eligible characters than X will still have to kneel all of those characters. In a melee game, each player may end up kneeling a different number of characters, depending on how many characters with a icon they control.
You may kneel as many of your characters as you like, even if your opponent cannot match your chosen number. For instance, if the opponent’s revealed plot is Valar Morghulis and Forced March is resolved first, you can kneel Ser Barristan Selmy (TS) even if your opponent has no character with a icon in play.
Neither you nor your opponent can kneel Ygritte with Forced March.