Need to increment serial number in read only grid?

You can use fv!index while looping. 
Or if it is a readOnly grid, then you can use enumerate() function. 

a!gridField(
  label: "Table List",
  labelPosition: "ABOVE",
  data: 'recordType!TableName',
    columns: {
      a!gridColumn(
        label: "S.No.,
        sortField: '',
        value: "",
      ),
      a!gridColumn(
        label: "Name",
        sortField: 'name',
        value: fv!row['recordType!Table.fields.name']
      )

},
)

In this grid listing I have to add serial number to the first column not the ID from the table.

a!gridField(
  label: "Table List",
  labelPosition: "ABOVE",
  data: 'recordType!TableName',
    columns: {
      a!gridColumn(
        label: "S.No.,
        sortField: '',
        value: "",
      ),
      a!gridColumn(
        label: "Name",
        sortField: 'name',
        value: fv!row['recordType!Table.fields.name']
      )

},
)

In this grid listing I have to add serial number to the first column not the ID from the table.