First strike When Knight of the White Orchid enters the battlefield, if an opponent controls more lands than you, you may search your library for a Plains card, put it onto the battlefield, then shuffle your library.
Knight of the White Orchid's triggered ability has an "intervening 'if' clause." That means (1) the ability won't trigger at all unless any one of your opponents controls more lands than you, and (2) the ability will do nothing if you control at least as many lands as each of your opponents by the time it resolves.
The Plains you search for doesn't have to be basic. For example, you could put a Sacred Foundry onto the battlefield.