I am so excited, although a newbie, at getting a tortoise. But I have noticed that most have many more than one .. is this recommended or that they are additive? Could I introduce another later - getting probably a Hermann
They are addictive. If you get a second, you must keep it entirely separately in quarantine for at least 6 months to make sure that neither makes the other sick.
It is not recommended to keep different species together as they often have different food and care needs and also they carry different bacteria. What is harmless to one soecies could kill another.
Introducing tortoises is your next pitfall. Many species are solitary and don't get lonely or want company. They bitterly resent a newcomer in their enclosure and you end up with fighting. This can be especially bad if there is a size difference. Bullies can be male or female, so the gender makes no difference to how territorial they are. Even if you do successfully keep them together, as they mature they may decide they don't want to share any more.
If you are addicted and decide to get a second tortoise think about it carefully. You may have to keep them separately forever... and tortoises can easily live for 100 years!
Many of us have more than one, however, each species has its own yard, and if we have more than one of the same species, they are usually outside in very well planted and safe habitats. Lots of sight barriers gives the tortoises places to get out of each others' sight and establish their own territory.
I have four species. Each species is kept in a group, but they all have their own space with no contact with other species or even other groups.
Groups of babies and juveniles are usually fine. Pairs usually are not fine. As they reach maturity, the sexes can become an important part of whether or not they all get along. In hermanni, two adult males are not likely to get along. One male and several females should be fine. Several females with no male will also probably be fine.
When you buy babies, there is no way of knowing the sexes without surgery, so if you get a bunch, be prepared to separate them all as they mature.