Thank you for your question.
If your broker (or other financial institution) left box 20 blank, you must still calculate it and report it.
You will need to verify the "Book Value" or Adjusted Cost Base for the shares/mutual funds sold and reported on the T5008. Otherwise, all of the proceeds are reported as a Gain on your tax return. Review your statements from the Investment Company.
Also select as Type of Investment "Capital Gains" under Box 19 on the T5008.
For more information, see the links below.
T5008 Slip – Statement of Securities Transactions
T5008 Statement of Securities Transactions – slip information for individuals
Box 20 – Cost or book value