I tried a lot by Field Calculator but it gives me
NULL as you show in table and another try it gives me value 1 in all rows.
Most of examples, on internet they are explaining old version and I have QGIS 3 and I am not sure how can I add id for all rows .
If I want to generate random id, how can I do it also.
I did as suggested in answer
But could i ask why the values are not looks like counter, it looks random. If i need to increase by one, how can i do that?