We don't set any limits. Since your designer is a real human, not a robot, they can only do one thing at a time. Having said that, nothing stops you from sending as many requests as you need and creating a queue/pipeline of requests.
If you have more than 1 design request in your queue, they'll work on them in order of arrival or priority (set by you). You can also ask your designer to pause a request and work on a more urgent one whenever you need to switch priorities, even if your designer is in the middle of an incomplete request.
Some of our clients have between 6/8 designers working for them on their tasks. Whenever you need more design work done at the same time, you can always add extra designers to your team.
