When Claim Jumper enters the battlefield, its triggered ability will check to see if at least one opponent controls more lands than you. If none do, the ability won't trigger at all. If the ability does trigger, it will check again as it tries to resolve. If there's no longer an opponent that controls more lands than you, the ability won't do anything.

For example, say you control three lands, one opponent controls four lands, and another opponent controls five lands. As Claim Jumper's ability resolves, at least one opponent controls more lands than you, so you search for a Plains card and put it onto the battlefield tapped. Now you control four lands, but there's still an opponent who controls more lands than you, so you repeat the process once, searching for another Plains. You end up controlling five lands (two tapped Plains plus your original three, which you probably tapped to cast Claim Jumper in the first place).