Many citizens (e.g. from the European Union, USA, etc.) do not require a tourist visa prior to arrival in Namibia. These travellers just get a stamp into their passport (which has to have two empty pages and needs to be valid 6 months after departure from Namibia) on arrival. Every tourist is allowed to stay in Namibia for 90 days per calendar year, irrespective of the number of trips, i.e. you can come e.g. 10 times for just 9 days or stay 2 times for 45 days. Double check at the port of entry that you get enough days for your whole trip plus a few just in case. Some tourists do not always get what they filled into the arrival/departure form!
Note: There is a theoretical option to get a tourist visa extended over and above 90 days per calendar year, however, in the last few years the Ministry of Home Affairs never allowed any extensions. And please remember, that you are not allowed to work with a tourist visa. Any voluntary work where you just get board and lodging is also considered work.
For further information please contact your closest Namibian diplomatic mission!