Why do i need to create a custome profile from a parent profile ?

One of the main reasons for creating custom profiles is that should one application require a tweak to some setting, say disabling Nagle's algorithm, if you only used the parent profile, then you would have to disable it for every application. Using custom profiles allows you to fine tune settings for only a single or a group of applications.

I usually create custom profiles that contain settings that can be used across a broad range of apps, and then create other custom profiles from there. iApps take this even further, and automatically create the necessary profiles.

There should be no real limit on the number of profiles that you could create. You'd likely run out of memory first.

Hope this helps.