I'm not sure what you mean by "incremented". Do you mean like an loop index?:
FOR index IN 1..100 LOOP
If so, you don't understand what a table index is at all. It is not a "counter" that gets incremented by anything. It is more like the index in the back of a book: a sorted list of keys and pointers to the information in the table.
The reason there are sometimes gaps of 20 between the numbers is because sequence values are usually "cached", and the default cache size is 20 - i.e. when you first ask for a new number using NEXTVAL, Oracle allocates 20 numbers to your session. If you call NEXTVAL again, Oracle gives you the next number from your cache. But if you don't call NEXTVAL again, the other 19 numbers are "lost".
For more info, read about sequences in the documentation.