There is no guarantee. Each Pokemon you encounter has a 1/8192 chance of being shiny. This number does not get larger or smaller for each Pokemon you encounter. Only shiny you are guaranteed to get is the Red Gyarados in Lake of Rage and the Black Haxorus in the Nature Preserve.
Note that 1/8192 does not mean that after 8192 Pokemon you will get a shiny. You could encounter a million Pokemon and never see a shiny. That's how probability works.