Sorry, currently FormsCentral does not support this.
Others may have a better solution - but in the absence of answers previously, I think of the response table as an Excel worksheet and do the following;
Once your form is created and distributed let the first couple of submissions flow in.
Now (as Form Author) view the responses table.
Look at the Menu on the upper right and choose Table ~ Add Column after
The new column wil be created with a default name like Column1 (Like Excel you can double click and change that name)
In the row relating to the first received submission, in the Column1 cell, enter the first number you want incremented e.g. 1
On the row relating to the second received submission, in the Column1 cell, enter the formula =sum(Column ID letter and Row Number+1) e.g. =sum(A1+1) and press enter
It'll auto populate the rows below with a number incremented by 1 (or your chosen increment)
Does the job for me
Now, let's stand back and await other ideas / suggestions! Good luck.
Thank you so much! That's perfect!