Valakut, the Molten Pinnacle's triggered ability has an "intervening 'if' clause." That means (1) the ability won't trigger at all unless, at the time a Mountain enters the battlefield under your control, you control five or more Mountains other than that new one, and (2) the ability will do nothing if you control fewer than five Mountains other than that new one by the time it resolves.
с магиккардсинфо.
Кому читать вломину, то нифига не случится, если на момент резолва тригера окажется недостаточно маунтинов на столе.