This entirely depends on what you want to do. All tortoise species make a fine pet when housed solo. I want to breed the species I keep, and I find it beneficial to my long term goals if the tortoises grow up interacting with other tortoises of their own species for their whole lives, so I tend to prefer groups of 6-10 tortoises housed together as babies. Then, as soon as I can tell the sexes, I separate males and females. Males grow up together in bachelor herds together, and sometimes have to be separated into individual enclosures if they won't get along. Females then get to grow up to a mature size in peace with each other without being constantly hassled and harassed by dominant amorous males. When the females are an appropriate size for breeding based on the species, I reintroduce a male or males, depending on the species and circumstances. SA leopards get one male for a group of females. Burmese stars can all just live together with multiple adult males and females in any combination. Russians are hit or miss however you try to do it.
I have seen difficulty in some cases trying to mix a tortoise that grew up all alone into a breeding group years later. They sometimes don't seem to know how to get along with their own kind. It usually still works out, but there is a bit more stress for the tortoise and the tortoise keeper. Females that behave inappropriately get sorted out by a big dominant bull male. Males that behave inappropriately usually receive no challenge from the females, and soon nature kicks in and they figure out what those females are there for, and it isn't fighting.
So generally, if you feel like you can make it work as they mature, I prefer groups. The downside is that if you get 6 Russian babies and 4 of them turn out to be males, you will have to maintain 6 enclosures as they mature, or sell off some surplus males. This is very species dependent, and also depends on your individuals. Russians tend to be very scrappy. Even females and juveniles will fight. Hermanni are better. In most cases, only adult males will fight with other adult males. Female Hermanni will sometimes get a little testy during egg laying times, but are usually amicable. The stars are the opposite end of this spectrum from the Russians. You can keep any combination of adult stars together, any mix of adult males and females, and no one wants to fight ever. All tortoises are individuals and there will always be unusual outliers, but these generalities usually hold true.
