Not following this, can you post some example data and then what you want in your new column?
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For example I got :
I would like to have a third column with, for each line, Modification1, Modification2, Modification3, Modification4...
Those datas come from a SharePoint List and and they are going to change and to grow, I want to be sure that if there is a new line added there will be ModificationN+1 on the third column.
I can't modified the SharePoint List and I don't want to use an excel sheets with the datas because I want to refresh it directly from Power BI.
If I understand you correctly, you should be able to follow steps below to add this column in your table.
1. Add an Index column in Query Editor under Add Column > Index Column > From 1.
2. Then use the formula below to add a new calculate column in your table.
Column = "Modification" & Table1[Index]
Note: just replace "Table1" with your real table name.