Oxy pot grows grow big. So I'd say you'd really fit 4 in 2.5x2.5. So 8 wouldn't be too much of an issue.
Some would say 6, but 4-5 weeks veg with LST you'll get away with 8.
8 autos 6 photos say. That'll give you as much veg time you want with photos and autos are autos go by the breeders height and structure and that.
Good luck