When you insert more than 999 lines into CCI, it gives a pop-up warning message as:

"Number of lines is more than 999 lines. You can adjust it by setting Skip Top/Bottom Rows values"

Below is the screenshot for reference:

CBSA RULE's

 According to the CBSA rule, the limit is 999 CCI lines per shipment.

You can’t send an ACROSS or IID shipment for Release with more than 999 lines.

If you want to insert more than 999 lines, you will need to break them up into multiple manifests.

However, accidentally if you sent the IID or ACROSS with more than 999 lines and if it is released then at the time of CADEX submission you can use the page break option.

Page Break - The page break gets around issues in CADEX allowing no more than 999 lines per page.

Below is the screenshot for reference-