Note the more registers at disposal of the tasks, the more you have to backup.
If you decide to limit your task number to 3 and assign a separate Rx bank to each task and limit their stacks to, say, 32 bytes each, they will be quite happy and you don't have much to backup. But if you decide each task has access to all the banks, can access the bit-addressed memory at will and have 80 or so bytes of stack for use, suddenly you have to backup a lot...