My Vadai barrel holds just a tad over 29 bottles worth of wine, so just under 6 gallons. I know this because I usually bottle straight from the barrel. Sometimes I rack to a carboy, but then I need another couple bottles to top up the carboy because the barrel volume is slightly lower. If the barrel volume is greater than six gallons, you will need topping wine for the barrel (or some other method, like marbles).
One important consideration is that you need to have a number of kits on hand to run through the barrel. The first few kits will really soak up the oak. My first kit I ran through the barrel only made it 3-4 weeks before it was really oaked. It's been bottled 15 months and it is still pretty oak heavy.
I've had my barrel since October 2013, and it is currently holding its 7th kit. So 7 kits in 16 months. This kit (CC Red Mountain Cab) has been in there for two months. The schedule for the first few kits went something like 4 weeks, 6 weeks, 8 weeks, 10 weeks...you don't want to leave the barrel empty, unless you want to mess with a bunch of sanitation and rehydration issues. So you need to have a few kits ready to go, or already bulk aging and ready to go, immediately into the barrel when the current barreled kit needs to be racked out.
I think you'll find you will have a lot of your own wine to top up with very soon. You may even have it already. If you do, it's kind of a bonus...more wine gets to rest in the barrel!
If you don't have a way to test for free SO2, consider investing in something to do so. The SO2 levels will drop fairly fast in a barrel.
Good luck!