Interesting question. You probably need to say whether you mean spoken, written or say internet-published text - or all of them. Wolfram Language can tell you for sure how many instances in a particular text:
Grid[{#[[2]], 
    Length[StringCases[ExampleData[#], "I don't know"]]} & /@ {
   {"Text", "DonQuixoteIEnglish"},
   {"Text", "AliceInWonderland"},
   {"Text", "OriginOfSpecies"}}, Alignment -> Left, Frame -> All]
