Ah - I would have called this "row numbering". I don't think it can be done with strata. If I absolutely had to, I'd try using an outer join and one of the aggregate functions, but I would use a separate wiki just to be on the safe side.
There might be a solution in another plugin by the same author, strata_templatery (which in turn needs templatery, also by the same author). Both plugins can be found in the github account of that author, and both work in hogfather. However, I don't know if the author intends to support those additional plugins for a very long time.