You can't - it is fixed at three. TurboTax knew about this issue many years ago and still hasn't fix it.
I don't know how CRA views this, but what I did was enter the top 2 countries and then lump the rest under the country code OTH, so that the total amount reported is at least correct. After I netfile, I then download the fillable pdf form in CRA website and file an amendment with proper country by country detail and mail in the amendment.
I find it unforgivable that TurboTax, having known this issue for years, hasn't fixed it. The CRA should not certify TurboTax for use until they fix this. I'll be getting a different tax software next year, if TurboTax still hasn't fix the issue. I heard a competitor has no problem adding additional lines.