you can not load test every scenario in you web application because there are time constraints.There must be some criteria upon which only those areas must be load tested which can cause issues .
On Which Condition Functionality should be Selected for Load Test :
- If client defined that a particular functionality will be used by X numbers of user now you need to validate that particular functionality at least bear load of X numbers of user mandatory and preferably you should test beyond that at least X+50 for having buffer.
- A functionality that is supposed to be used frequently like there is a web app and every new user will have to first register and then can access its content. So it is mandatory to test performance of registration page.
- A Set of Complex functionality which consumes most of the server resources . you need to load test these functionalists too because it may happen when even 10 users perform these operation you server would crash.