The way you would do this would be to use the auto-increment feature. First find the existing qtree's and ensure the filter has ordered it to find the last one first. Then use the auto-increment feature as follows:
Select the "..." next to the new qtree's name:
Find the existing qtree's (in this case by name prefix in volume):
Specify a default number to start off with like 0001 and increment by value:
Update the filters if you want the number to be unique across all volumes within a controller or by some other mechanism. It just needs to find the highest existing value.
Note: This way does mean if you create then delete the last-number it will be re-used.