ok, well, I'm going to correct a little of your terms.
"Auto" implies that a field in a table is set to "auto Number" type.
In that circumstance, EVERY RECORD gets a sequentially higher auto number.
I would like to make "sr.no" field auto populate when quantity field is updated.
In your usage, I think what you'll need to do is write some code that determins the next highest sr.no number for THAT specific quote but NOT do it "on update". if you do it on update some action will take place even if the qty is just CHANGED instead of added new.
For example, your sales person sets a new (blank line) qty to 5 and the new (sequential) sr.no is generated for that line. If you have it set to triger "OnUpdate", if the sales person changes the qty from 5 to 7, yet another new sr.no number will be generated.
Some others may differe on this, I don't know, but for me, i have an invoicing system I've built where all the invoice line items 9for EVERY INVOICE) are in one seperate table. Each line item has its own unique ID (AutoNumber), but there is ALSO another field which denotes the exact line number for that record in the actual invoice for which that line is associated.
To be VERY CLEAR, this absolutely would not be needed if I did not want to have the line number visible on the acutal invoice.
If I were happy without a line number on the invoice, all I would need to do is sort the line item records for that invoice by ascending Autonumber and they would line up correctly.