If the line item quantities are correct, select Receive All and you will not be prompted for SN entry.
If the quantity was backordered, simply skip that item (default is quantity zero).
If the quantity is short, and you want to enter serial numbers later, click on the quantity received field, then click on the button with the ... ellipsis (yep, that's what it is called), then click Add+OK for as many items as you receiving.
Example: If you ordered 5 and received 2, click Add+OK twice. Notice the Total will increment in the upper right corner of the SN Detail box as you do this. Now click OK and 2 out of the 3 will be received (with the discrepancy noted in red).
OR -- Click Receive All and go back to the line items that needs to be reduced, click on the ellipsis, and delete entires until the count is correct, then click OK.
Example: Receive All inserted 10 but you only received 8. Delete two of the blank serial numbers under the ellipsis and click OK. The received quantity will now be 8.