This is from the full OED:
b. In singular. A countless number of specified things.
1869 ‘M. Twain’ Innocents Abroad xxx. 321 Throw a stone into the water, and the myriad of tiny bubbles that are created flash out a brilliant glare like blue theatrical fires.
1961 B. James Night of Kill (1963) ix. 104 The hour which, like a spade turning clods of earth, exposed to the day a myriad of busy creatures that had laid dormant in the quiet night.
1987 Observer 20 Sept. 46/4 A myriad of small, specialist software companies have also been spawned in the new ‘sunrise high-tech’ areas.