What is the use of haroof yarmaloon and how many are there in number?

Question
What is the use of haroof yarmaloon how many in number?

Answer
The group of letters that form the word   are six in number.  The letters are: The ,, and the .   They are significant when any of these six letters follow a saakinah (vowelless) or tanween, the saakinah or the saakinh of the tanween merge into these letters, which is called in Arabic.  Please see the following link for full explanation of this rule and the role of the letters in the group

http://www.abouttajweed.com/the-noon-saakinah-and-tanween/merging-of-the-noon-saakinah-and-tanween-p-2.html