Barrels are topped off as a matter of regular practice to avoid oxidation. It makes no different if you have 1 barrel or a 100 the process is the same. All containers of wine stored should have no head space. I use a 5 gallon Cornelius Tank Charged with Nitrogen and the tank has a hose and a spigot to reach all my barrels. As the wine level decreases in the tank it is replaced by nitrogen keeping it safe. Many Wineries use this type of system to keep barrels topped off. As far as practice is concerned some like to top once a month others believe that opening the barrel at the frequency will introduce too much oxygen depleting the SO2 levels so they are in favor of topping every 3 months. I usually top between 6 to 8 weeks and test for S02 every 6 month. If you leave a barrel for a month and have a tight bung you can hear the air enter the barrel as you crack the bung open. This vacuum also protects the wine as Air does not enter the barrel until you open it. It is a bit strange that while some o2 is transferred through the wood of the barrel to micro oxygenate the wine the barrel still creates a vacuum. Even more to consider is the humidity level if below 60% in the Cellar will cause the wine in a barrel to lose water if above 60% the wine will lose alcohol.
Malvina
