here is my problemI have this code to make an autoincrement variable in oracle database:
CREATE TABLE Korisnici
(
id_korisnika number PRIMARY KEY
It's not a problem. You have probably specified cache 10 in your sequence creation script. If you change this to nocache it will help with the gaps, at the cost of a hit to performance, but they'll never go away completely as any rollbacks done, and killed inserts etc will use up values. Please see this Ask Tom post.
At the end of the day it shouldn't matter in the slightest. If you're relying on an unbroken sequence as the key of your table then you probably have a problem with your data rather than sequences.