From the Kew Website:
The seed collections in the Millennium Seed Bank constitute the largest and most diverse wild plant species genetic resource in the world. The great majority of this collection has been collected by the associated global network, the Millennium Seed Bank Partnership (MSBP), which is active in over 80 countries and is the largest ex situ plant conservation programme in the world.
The purpose-built facility at Wakehurst Place is based around a vast vault for the long-term storage of seeds for research and conservation. Following collection in-country, seeds are prepared and dried (to around 4–6% moisture content, fresh-weight basis), before storage in deep-freeze chambers (-18 to -20°C) within the vault; following international standards.
At present there are more than 80,000 seed collections in the bank; representing over 37,600 species, from almost 5,800 genera and more than 330 families. That is, at least one collection each of around 12.5% of those seed-bearing species estimated to have orthodox, bankable seeds.