Is there a dollar threshold that triggers CRA reviewing TFSA accounts for day-trading activity?

No, there is no published bright-line dollar amount or transaction-count threshold that automatically triggers CRA reviewing a TFSA for day-trading activity. Instead of a fixed number, CRA looks at the overall pattern of how the account is being used - factors like trading frequency, how short the holding periods are, the account holder's market knowledge and experience, and how much time is devoted to trading - to assess whether the activity looks like carrying on a business rather than ordinary investing.

This means there's no safe number you can point to and assume your account is below it, so it's fine. A relatively small account traded very actively and frequently could draw more scrutiny than a much larger account that's simply grown through long-term holdings with occasional trades. Because the assessment is genuinely about the pattern of activity rather than the size of the gains, focusing on how business-like your actual trading behaviour looks - frequency, holding periods, and time devoted to it - is more useful than trying to stay under some assumed dollar limit that doesn't actually exist in CRA's published guidance.