I usually end up with a bit over 30. There's definitely some variation from carboy to carboy, but out of the 14 that I have there's never more than about half a bottle's difference.The carboy way topped off. Not sure where I lose it but have never got 30 full bottles from a batch. Usually 28 and just short of a 29th. I do have room to bulk age, just need to get an additional carboy.
Keep an eye on your local craigslist for people selling winemaking supplies. You can usually pick up carboys for less than half the price of new.