If you do the "standard" east coast Africa route, your maximum range will need to be about 300kms or so, which your 20 litre tank should almost just do. The two sections of the route with the longest fuel stages will be northern Sudan from Wadi Halfa heading south and then northen Kenya.
If you take the Nile route from Wadi Halfa in northern Sudan, fuel should be available in jerrycans from one of the small villages (Abri) or from a petrol station near Argo, meaning you could probably get by with a range of only 250 - 300kms if they have fuel. If you plan on taking the railway tracks route south from Wadi Halfa you will need a range of 370kms according to the Michelin 745 map. I haven't done the railway tracks route, so will leave it for others to comment if Michelin is accurate.
The fuel stops in northern Kenya via the traditional Moyale-Marsabit road are about 250kms. If you plan on going via Lake Turkana you will need a bigger range.
Further south fuel is readily available.