It's on my 'to do' list to look into but I don't think it's to do with number of eggs but rather how you sell them. If you sell them direct to consumer from the farm gate or otherwise, you don't need to register. If you want to sell via other outlets (e.g. local farm shop) or to businesses who will use the eggs in a finished product, you need to register. The reason I'm hesitating is partly because I believe you have to grade the eggs if you are registering and I have a small flock, many of whom are different breeds producing very different sized eggs so that would be a nightmare. Also I think the stamping is quite a hassle - there was a post on here about the cost of the stamp too.
H