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]
