This surprised me, so I've just looked it up. I'm now surprised for a different reason.

"Webster's New International Dictionary" contains "more than 600,000 entries" and it's a single volume, albeit a hefty one.

"Oxford English Dictionary" on the other hand, has "301,100 main entries" and yet the last paper version was 20 volumes. Presumably because OED contains more details per word.

Of course, for anything but a spelling bee it's pretty absurd to compare dictionaries by number of entries. Beyond a certain point: you're just buying more chaff like family names and alternative spellings.