Why are there are multiple kaddishes at the end of morning tefillah (at least 3-4)? Did people only used to say one? Why the need for so many?

The Kaddish after Tachanun & the Kaddish after Uva Lezion are for the cantor as a part of Seder Hatfilah. The Levush says that the reason for all the other Kaddeishim that come after Aleinu, Shir Shel Yom etc. are because it is necessary to have a Kaddish said after each time Psukim are said, and the Rabbis instituted that these Kaddeishim should be said by orphans because some orphans are not able to lead the services and it is a great merit for the departed parents soul when their son says Kaddish. [לבוש סימן קלג ורמ"א קלב ב ובמ"ב ס"ק י].