Make it easier for everyone to track the status of tasks by sending tasks one email at a time, and make the task it requests very clear. When a person has completed a task they just have to reply "Done" to that email, delete the email from their Inbox and then move on to the next task.
This rule can be by-passed when dealing with small tasks relating to the same topic. In cases where this is needed, you should number each task that you wish to be completed, as per "Do you number small tasks related to 1 topic". however, the person completing the task should still reply a single "Done" to the whole email once they have completed all of the small tasks.
Also send separate emails per topic - that way there can be one email per topic. The advantages are that you get an email history on a specific topic and it is easier to include someone else.
Figure: Bad Example - One email for multiple separate tasks. 
Figure: Good Example - Separate emails for separate tasks. 
Figure: Good Example - A few related tasks in one email.