Hello from me in South Lanarkshire
The number, I'm assuming, is a CPH number. County Parish holding number. You'll not be able to get any livestock without it. If your land has previously been farmed in it's own right (and not as part of a larger farm) it should already have a number allocated. In scotland you need to contact your local SEERAD office who will be able to help you, the number should apply to ALL your land, not just the grazing (cos in theory you could have pigs forraging in your woodland
) Then you'll need to contact your local animal health office to register for your herd numbers - for whatever animals you choose to have (but not needed for chickens - i think). There is an animal health office in Galashiels, their number is 01896 758806
Hope this helps, look forward to hearing more from you
Karen x