For each wallet connected to the staking interface, you can have 1 of each schedule (1 week, 4 weeks, 52 weeks) staking at once. We use bank deposit logic in the staking system to keep everything as secure as possible. This means that if you go to stake additional tokens into an ongoing stake, the timer will reset on that original amount too.
It is important to note that the rewards earned up to that time you add more will be saved and counted into total rewards at the end of the duration.
Example: John puts 1000 $EMBR into a 4 week staking schedule. 2 weeks into that schedule, John decides he wants to add an additional 1000 $EMBR and adds the additional 1000 $EMBR into the 4 week schedule. This will reset the timer on that 4 week stake, but the rewards earned in that first 2 weeks will be saved and added into the total rewards that unlock at the end of the reset timer. At 6 weeks, John would have 6 weeks worth of the 4 week staking rewards.
Example: John puts 500 $EMBR into a 1 week stake, 500 $EMBR into a 4 week stake, and 1000 $EMBR into a 52 week stake. Because each stake is on its own schedule, there wouldn't be any reset timers, so long as John doesn't enter additional tokens before any of the durations are up.