Selleers can add items as and when they wish, 24 hours a day, 365 days a year.
There is no limit overall as to how many items dealers can upload to the HV. There is a delay function so if you want to go in once a week, you can set your items to show over that period, or longer. This is so that everyone gets a chance to be seen, rather then a few sellers having an overbearing presence on the site and to give smaller dealers as good a chance as the larger ones.