Unfortunately, TurboTax Online only supports entering 15 rental properties.  You can file with your 15 rental properties entered and file an adjustment to add the other 5 properties using your CRA account online.  See the CRA website for how to change your return: https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/change...

If you are willing to wait for an available tax expert to become available, you can use TurboTaxLive Full Service option.  https://turbotax.intuit.ca/personal-tax-software/live-catalogue.jsp