Trello – way to track the number of days a task was worked on in Trello

There's currently no way to do this directly through the user interface. If you feel comfortable with some light scripting, you can automate the board creation and card copying by using the Trello API, but otherwise, your best bet is to make a feature request.

The "why" is that Trello didn't used to have a separate "subscribed" and "assigned" concept. Notifications were generated for cards you were assigned to, and there was no way to "unsubscribe" from a card except to unassign yourself (and no way to "subscribe" to a card except by assigning yourself).

This was limiting, so we added a separate notion of subscribing and, because it's usually what you want, set it up to automatically subscribe you when you're assigned to a card (as a nice touch, this was consistent with how it worked before, so people who were used to the old behavior didn't have to change their workflow).

Unfortunately there's no way to change this. The solution to the notification woes is, sadly, "don't do that." If someone doesn't need to be updated about everything that happens on a card, using @mentions to send them notifications when they do actually need to chime in is a better idea.