If a branch represents a summary task (i.e. a task with sub-tasks) you will find its values are not editable, that is because summary tasks have their values calculated based on their sub-tasks.

As an example, if a summary has two sub-tasks that are linked to start one after the other, each with a duration of 2 days, the summary task will automatically calculate its own duration to be 4 days.

If you consider the map view, when you view your map as a Gantt chart parent branches are converted to summary tasks and branches with no children become tasks. If you switch back to the map view temporarily you should see the dark grey summary icon (downward-pointing shield) against those tasks that you can’t edit.
If a summary task represents something that you need control over it should be a task in its own right.

There is no limit to the number of tasks MindGenius can support, however the power and resources available to your PC, combined with the complexity of your project structure, e.g. Number of task links, time constraints, etc, will have an impact on your project’s performance.

If you feel performance is suffering you can make use of the map explorer to focus in on areas of your project. The map explorer is collapsed by default while you’re viewing the Gantt chart however it can be expanded and used in the same way as you would in map view.