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.