assumed you have a repeatable subform "SF" which contains a numeric field "Nmb".
You can update the instance count with a small script (FormCalc) in the indexChange event of your subform.
form1.#subform.SF::indexChange - (FormCalc, client) Nmb = $.index + 1
This makes no big difference.
Nmb.rawValue = this.index + 1;
It worked....I had to add a couple of parents in so it was this.parent.parent.index +1 just for other people wondering why it might not work. I put the script in the layout:ready event location.