If your T5008s have been imported to your tax return using the CRA AutoFill My Return (AFR), you will need to go in and "edit" each slip individually to enter the Cost or Book Value in Box 20.
The following FAQ from TurboTax explains how to enter your T5008 slips in both the Online and CD/Download.
How do I enter a T5008 slip (and matching Relevé 18, if applicable)?
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