how to create test plan for regression code example

Example 1: how many test case in your regression

It depends on the project.
In my current project out of 1200 test cases 
in regression suite around %70 is automation.
We use 6 vm to perform parallelexecution it takes
3 hours to execute depends on the test data.

Example 2: how to create test plan for regression test

in Jira test plan is nothing but grouping tests
(and labeling them )so that they are regression,
smoke or both. All you need to do is create
a test plan name is Regression and add tests
to this test plan by linking those tests with
the test plan you have just created.
So question which scenario goes to regression:
we decide it together as a team, mostly
positive scenarios go to regression,
the ones that needs repeated execution,
retesting once a new build is added.