you need to perform regression testin before delieverina change request for an apllication. whatare the basison which you will test casef for the smae code example

Example: 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.