In one of the projects we have 18 stories but where are as in portfolio plan I am able to see only 12 stories I am missing 6 stories in the plan. In the current plan I have not specified anything in the filters.
Even if you've not applied any filters yourself, Portfolio automatically applies filters to your plan. These are Completion date and Schedule range. These filters limit how far back in the past and forward in the future the plan looks. You can read about these here https://confluence.atlassian.com/jiraportfoliocloud/using-scope-filters-828785257.html.
These filters could be hidden by default. You have to click on More to display these filters.