List items maximun

It is also worth noting that the real maximum is the index number at 999,999,999. If I create a new list and then add one item the index for that item is #1. Then if I delete that item and create a new item the index of my next item is #2. #1 would not be able to be used again until I reset the index.

Therefore if I have 0 items in my list and my next index # available (because the rest have been cleared) is #1000000 then I am not able to add 999,999,999 items as the first 1,000,000 Index numbers have been used. 

You can Reset the Index Number manually, but the true maximum number of items you can add to a list at a given time = (999,999,999 - Current Index #)

The other factor here is that a list member is 500 bytes, so there may be a size constraint within your workspace. At 999,999,999 items and 500 Bytes a Piece, that gets to almost 500GB which would not fit within a workspace at this point. You would also want to factor in anywhere that the list was used as a dimension to find the amount of size that the cells are taking as well. 

https://community.anaplan.com/t5/Anaplan-Platform/Calculation-of-modules-amp-Size/m-p/44695?_ga=2.190928297.2035036762.1586793311-1876461626.1579557669#M5300

I hope this helps!

Jason