Immortals have a choice of 11 different task types and need to complete 60 per week. Some of those tasks - Bestiary hand-in and Bounties - are worth 2 sigils each. The rest of the tasks are worth 1 sigil each. So, what Immortals would do, to maximise the amount of sigils earned per week, would be to do 5 bestiary and 5 bounty tasks per day, without doing the other tasks, or at least doing those tasks and not collecting it, so they didn’t get more of those 1-sigil tasks.
Since a recent patch, these tasks now stack up to five, i.e. previously if you did a dungeon run, the task for dungeon would be done. Now, this will stack up to five times, so that if you do 5 dungeons, you’ll get five stacks of this Immortal task. This means it’s no longer possible to stack just the bounties / bestiary tasks because the other 1-sigil tasks now take away more of the 60 possible tasks.