Cary Fowler - who heads the trust that oversees the seed collection, which is 620 miles from the North Pole, said the facility now houses at least one-third of the world's crop seeds.
"In my lifetime, I don't think we'll go over 1.5 million. I'd be rather surprised if we go over a million," Fowler told The Associated Press. "At that point, we'd have all the diversity in the world ... and the most secure samples."
Located in Norway's remote Svalbard archipelago, the Svalbard Global Seed Vault is a safeguard against wars or natural disasters wiping out food crops around the globe. It was opened in 2008 as a master backup to the world's other 1,400 seed banks, in case their deposits are lost.
War wiped out seed banks in Iraq and Afghanistan, and another bank in the Philippines was flooded in the wake of a typhoon in 2006. The Svalbard bank is designed to withstand global warming, earthquakes and even nuclear strikes.
Despite the rapid progress, Fowler said the bank still has significant holes in its collection.
"There are a few unique collections that we don't have up there yet - Ethiopia and some of the Indian materials and some of the Chinese materials," he said.
The most recent additions include a mold-resistant bean from Colombia and a collection of nearly every agricultural soybean species developed in the U.S. in the last century.