How many reign as king of Scotland throughout the play?

Technically, there has only been one King of Scotland (John Balliol). The rest have been Kings of Scots. This is because Scottish kings were kings of people rather than the land, which belonged to God

Leaving your god aside for a moment, I make it 34 kings between Donald Bane 860 AD until King James VI 1603, when he became King James I of England as well. List is at www.scotlandinaweek.com/kingsofscotland.html

From Kenneth McAlpin in 843 to Queen Anne in 1707 there have been 50 monarchs of Scotland/Scots.