If you are using TurboTax Premier CD/Download, any returns that you prepare for a taxpayer with a net individual income under $25,000 will not count towards the 12 returns that you have paid for. However, if you NETFILE the return, it will still count towards the CRA's limit of 20 returns that can be filed online.
 

If you are using TurboTax Premier Online, then there is no income limit. You pay per return regardless of what the net income of the taxpayer is.