Sequences go up to 9. In the event you have more than that - exit out. It’s over.
Kidding. In all likelihood - if you need to use more than #9, the planes behind #9 will be far enough out that you can delay their sequence. Just give them a pattern entry, and start sequencing as soon as a plane lands and #9 becomes available again.
Additionally - that occurrence is pretty rare, especially on the expert server. When it’s that busy, APP will typically be active. However, I have had it happen a few times. In those instances, I also had to issue 360s to keep the traffic properly spaced. When planes call-in from 30nm away, a sequence is not often needed/practical, because so much can happen in that time. So as the planes come closer, and the picture becomes clearer, you can start issuing sequences that won’t have to be amended multiple times.