Hi Swetha,
Add one more attribute in the entity like formatedSNO.trigger a microflow after creating object or before commit event handler or a custom microflow where ur commiting the object.
use a change object in the microflow in the change object create a string with the autonumber which format u wanted(adding comma) then assign that string to the formattedSNO attribute.
This will work if u still have doubts please share the format number u needed like split logic u implementing.