Can you tell me how many Kings and Rulers there were throughout history?
The definition of a king or ruler is quite flexible, ranging from monarchs, to tribal chiefs, to modern day elected officials. Additionally, not all kings and rulers have been documented by historians, though there is better documentation in certain geographic regions. 
If you tell me what you mean by kings and rulers, I could give you a rough estimate through.
Let's limit it to rulers that weren't elected, have names and are verifiably documented in books or other forms of writing.
That greatly reduces the number of rulers throughout history. According to the Encyclopedia Britannica, there were approximately 3,000 monarchs ruling in Europe between A.D. 800 and 1800. Additionally, there were a number of other rulers throughout the world, including China and Japan, during this same period.