I made little experiment. After Whites captured Novorossiysk I disabled AI and watched how long Denikin Army can survive in small town with no direct connection to friendly territory.

You may see that Novorossiysk is small town with harbor, surrounded with regions in which Reds have 100% military control. As, according to Wiki, supply may only transit through regions in which a player has a minimum of 25% Military Control, we may assume that White Army is practically isolated and dependent on what it can be provided within a Novorossiysk region. Tooltip says that Army consumes 50 supply points per turn. Now, if I understand correctly what is written in Manual, each size 2 town with size 2 harbor can produce 24 supply points per turn (eg. town size 2 x 8 + harbor size 2 x 4). So, we may expect that total supply stock in Novorossiysk starts to dwindle by 26 supply points per turn (50 needed minus 24 produced).
As you may see from the table in screenshot, that's not what is happening. Supply production is oscillating (?) but average supplies are not going down as expected. May someone please explain what I'm missing here?