Seeing you have several fields, it really is a job better suited for ArcPy and an update cursor rather than the Field Calculator. That said, if you will only do this once and be done, then maybe a slightly clunky and manual way with the field calculator makes sense instead of learning how to script.
I believe the following will work, just increment the numbers starting with 0 for each new field:
!MaintenanceTasks!.split(',')[0] if !MaintenanceTasks! and !MaintenanceTasks!.count(',') >= 0 else None
For example, Task1 will use 0, Task2 will use 1, Task3 will use 2, etc....