By a Private Military Company, a Paramilitary Organisation or even a part of the regular Military

What is a mutiny?

Wikipedia says: Mutiny is a revolt among a group of people (typically of a military, of a crew, or of a crew of pirates) to oppose, change, or overthrow an organization to which they were previously loyal.

In this case the organisation is the Russian State.

The size of the group mutinying must be at least 500 people. (this could either be the headcount of the organisation (if the organisation is generally involved in the mutiny) or the reported number of participants in the mutiny.