It is not known how many people are Bitcoin mining. It is estimated to be in the thousands.
The number has probably been going down as cpu mining is probably a money-losing proposition, even with the increase in Bitcoin price. The same is becoming true with graphics cards. This is because newer, fast mining hardware is coming on line. These are called ASICs for Application Specific Integrated Circuits. They are designed to do one thing very efficiently. As more systems come on line the “difficulty” increases so it is harder to solve the problem. This is so that a new block is found every 10 minutes on average.
Note that sometimes people think the number of Bitcoin nodes is related to the number on Bitcoin miners. However, many people run Bitcoin-QT without mining. Also, those that mine via pools do not need to run Bitcoin-QT because the Pool does that function on a remote server. There is no direct relationship between the two numbers.