I'm not sure what your end goal is, whether you're trying to come up with a unique ID for your transactional data, etc etc.
But if you're trying to derive unique ID using Anaplan, then our experience with other clients tell us that this is a solution which is unsustainable and performs poorly [This goes against the 'P' & 'S' within (P)LAN(S)]. The reasons are:
1. There'll be a lot of TEXT formatted formulas and line items. Anything 'TEXT' is bad for performance. While TEXT is unavoidable, we should still do our best to limit the usage of TEXT.
2. I'm not sure how big your Transaction list will grow, but a module with USERS x Month x Transaction list will be huge.
Please check out myth 3 in https://community.anaplan.com/t5/Blog/The-Truth-About-Sparsity-Part-1/ba-p/44493
But in anycase, do see if the following works:
1. Number = VALUE(MID(NAME(ITEM(#Transactions)), 2, 10))
2. Rank = RANK(Number, ASCENDING, SEQUENTIAL, ISNOTBLANK(Metric))