Pretty sure that you could fit 6, 5 gallon glass carboys. The 6, and 6.5 are too wide to fit next to each other, and have to be staggered. But the 5's fit nicely side by side.
In this picture, I have (Maytag 14.8):
3 - 5 Gallon Glass Carboys
1 - 6 Gallon Glass Carboy
1 - 6.5 Gallon Glass...