If you would like to have a sequence number not related to the auto number of your used Entity, you could create a helper object in form of an inpersistent entity and you need to build your list on that base. Then you could add an attribute for your sequence number and iterate that number in the list on the inpersistent entities. Small hint: in microflow created lists, you don't have all possibilities, like filter functions for example, as in a xpath list.
You can add an attribute for this in your entity. Then in the datagrid show this attribute as a column.
The integer or autonumber data type would work. Autonumber will automtically be set and will be incremental.
Here is the documentation on data types.