The serial number is up to you
You can pick the serial number you want - you are the manufacturer. I chose to use my "Builder Number" and not the serial number van uses to keep everything coordinated or a totally different number of my choosing. I contacted Van's near the end of my 8 year building period and they issued an FAA bill of sale for the kit (when you read the FAA requirements you will see they provide for that). The EAA documentation on getting through the "FAA please make my airplane legal to fly paperwork process" was VERY helpful to me. It is complex and sometimes mind bending but if you read that information, go on-line to get the FAA documents directly and read them, reconcile what the documents from those two organizations tell you, decide on a plan that is going to give you what you want in the end (the repairman certificate was very important to me), then go through the serial process (you cannot do it in a single step) and collect all of the documents along the way, it works out very well. This is something to focus on, get it right and be done with it. The airplane manufacturer not the kit supplier assigned the official serial number that went on the ID. Plate when I started my process in 2003. Things change so you have to go to the source and follow the rules that are in effect now.
Bob Axsom