The noun referring to (1) an ample amount or quantity or (2) the condition of being full or ample is plenitude, with only one t. The misspelling plentitude is so common that it’s been accepted by many dictionaries as a variant spelling. But the root of plenitude—and of plenty—is plenus (meaning full), without a t.
Both forms are common in publications that tend to reflect popular usage—for example:
This seems too harsh, yet one aspect of the Spurs experience which Arsène Wenger is sequestrating is the plentitude of false dawns. [Telegraph]
No theme pops up as often in protest signs as anger at David and Charles Koch, the industrialists who fund a plenitude of libertarian think tanks. [Slate]
But despite the plentitude of grocery stores and bank branches and retailers, Mr. Oliva objects to the idea that the neighborhood isn’t lacking. [The New York Observer]
I used to think that because he left school at 16, my father was a victim of the sheer plenitude of work available after the war. [Guardian]
But Mr. Ohlson had other ideas, inspired in part by a plentitude of flat terrain and open sky. [New York Times]
The Cretian resorts are small villages that have been transformed to enter the 21st century, given their plenitude of shops, bars, restaurants and amusements. [University Observer]