We should also note that the "legion" (Latin: legio) is one of the basic units in the Roman military system. So, when the demonic presence identifies itself as being named Legion, this alludes to Roman military term.
Roman legions were divided into 60 "centuries". As the root cent- indicates, a Roman military century, in theory, could have 100 soldiers. Thus, a Roman legion could, in theory, have as many as 6000 men. In practice, however, Roman legions seldom had that many active soldiers in each century and most scholarly sources I have seen argue that the typical Roman legion probably numbered between 4200 and 4800.
Mark's text at 5:13, though, may give a more precise hint at the number of demons in this particular "legion", since in this verse the Gospel writer tells us that the herd of pigs into which Jesus sent the demons numbered about 2,000.