I think the issue is this:

Add a serial-numbered item to a sales order, with qty higher than 199.
When delivery is created, it splits the lines so there is a qty of 1 on each line, since each individual unit requires a separate serial number. Therefore, if the qty on order is more than 199, it would need to fill the first delivery with 199 lines, and then add remaining lines to a new delivery, and so on until there are enough lines to cover the qty.