Can just one developer create a company for his game ?

Is it possible for just one game developer to create a game company to presents his game or do you need more than one developers to create a company with a name and if it’s possible to create a company is there any way find a name that’s not is use ?

For each of your question i can only tell you to inform yourself in your own country. The founding of companies has certainly nothing to do with Unity and might vary from country to country. Usually there’s no problem with one-man-companies. It’s also a matter of what company form you want.

The name problem (also off topic) is a similar one. You usually can use names that are already in use as long as it’s not related to the same thing. But you should check if it’s a registrated trademark or otherwise restricted. Those “tests” can only be done on concrete names.

So just search, if you don’t know how, ask google how to use google properly. Finally each country usually has offical agencies / administrations which can be ask in such cases.