Hello there,
1. Namibia is a visa exempt country.
2. You can stay how long you request, however it is up to the officer to determine how long you could stay.
3. You have to convince the officer hat you have strong ties to your country & that you will leave at the end of your visit. They don't want to risk you overstaying. And a criminal offence & ill health will affect you visiting as well. A proof of funds to show them that you can support yourself while in Canada is also one of the requirements.
I would recommend checking the CIC website for more in depths information.
Hope I helped