Former botanist here.
It is not uncommon for this sort of behaviour to happen in the botanical world. There are many forms of this sort of thing, the most commonly seen one is coppicing. Coppicing is often done by people, but it also occurs naturally. In the natural case, an old tree (or a young one) falls over and each branch starts to grow upwards, putting down roots at the node with the parent trunk. Eventually the parent trunk dies and rots away, leaving an stand of trees that are all genetically identical, usually in an elongated ring around the shape of the trunk. Some of this form of growth is a result of the shape of the original tree and the rest is a result of competition for things like light.
There are also some species of tree like the Quaking Aspen (Populus tremuloides) which grows in vast genetically identical groves. In this case the species propagates through its roots.
To exactly answer your questions:
- It varies, I don't know if there is an upper limit, but the lower would be 1.
- No, you can grow them perfectly happily as individuals.
- See above answer.