You can list as many as you like, as long as you don't have any listing or item limits, it's just that the first 600 are free, and after that they will cost you an insertion fee for each one. (To clarify, listing limits affect how many listing you have, item limits affect how many individual items you can have available, so one listing with 10 of the item available is 1 off the listing limit, and 10 off the item limit).
