I'm a ninth grade student and I don't understand how carbons catenation property allows the formation of many millions of organic compounds.My doubt is there is no need of linking with the same atoms to form many different compounds.
By 'catenation property' I'm guessing you mean the ability of carbon to form long chains? This is one of the reasons for the large number of carbon compounds, although it may not be the main reason (or it may, I just don't know).
Consider that it is possible to have molecules with 21 carbon atoms - the 'heneicosanes' apparently - and that just the number of possible tree-like molecules is 2,144,505 then we are already getting into the millions.
Now consider that we could have double or triple bonds anywhere in those molecules, along with atoms of other elements (O, N, S, etc) and you should see why there are so many possibilities. In contrast, you only get sulfur chains up to around 8 (?) atoms.
The other important property of carbon is having up to four neighbouring atoms. Although that's a bit harder to show, the number of possible arrangements is much larger than for elements that can only have two or three. It can be helpful to just try drawing out all possible structures for a small elemental formula (like C4H10).