If you need to show the serial number in search screen you can do so using the following snippet.

[column new label='Sr. No.' cond='{form_data.submit_handler}' equal='datatable']
   [calc raw]
     [module.set temp_calc='{module.dataset.pagesize}*({module.dataset.pageno}-1) + {env.@my_row.index}' /]
     [module.get temp_calc.math /]
   [/calc]
 [/column]

Please remember this snippet will only work in the data table, and not in CSV download.