Do I calculate the tax when I receive the mining reward or when I sell it?
Both of them. When you receive the mining reward you should calculate the tax as "Inkomst av tjänst". When you sell it, you should calculate "inkomst av kapital".
Does the tax depend on the tax of my regular income (30%/50% etc)?
The "inkomst av tjänst" depends on you regular income. The "inkomst av kapital" does not. However, "inkomst av kapital" may depend on "utgift av kapital".
Is there any advantage of creating a company (enskild or aktiebolag) do run the activities?
This is complex. Do you own the company to 100% yourself? In what jurisdiction is the company located? Do you have both active and passive income from the company? You should probably consult a lawyer.