How to name long hydrocarbon chains?
Use the table mentioned in the linked article, §NT-1.1:

1   mono- or hen-*  10  deca-       100 hecta-      1000    kilia-
2   di- or do-*     20  icosa-**    200 dicta-      2000    dilia-
3   tri-            30  triaconta-  300 tricta-     3000    trilia-
4   tetra-          40  tetraconta- 400 tetracta-   4000    tetralia-
5   penta-          50  pentaconta- 500 pentacta-   5000    pentalia-
6   hexa-           60  hexaconta-  600 hexacta-    6000    hexalia-
7   hepta-          70  heptaconta- 700 heptacta-   7000    heptalia-
8   octa-           80  octaconta-  800 octacta-    8000    octalia-
9   nona-           90  nonaconta-  900 nonacta-    9000    nonalia-


just enumerate the digits of the carbon and add "-ne".

So $\ce{C21H44}$ becomes "hen-" "icosa-" "ne" = henicosane / heneicosane (the latter is just a spelling variation).

There are exceptions like propane ($\ce{C3H8}$) but these only occur for small molecules, up to $\ce{C4H10}$.