Amount of orcs needed varies as per samidodamage's post above, as such so do the number of armories needed. For me 2 armouries was plenty for a while, I eventually added more but it had nothing to do with orcs but with training troops. Now I have 6 armouries and my orcs numbers are in plague proportions, nothing short of a war is going to get those guys back under control.
In terms of numbers (like with other buildings) you want to be increasing your number each week, so just monitor their growth from one week to the next. This factors in your natural catering/negotiating. Then you just have to decide if your surplus is looking enough for your needs and that comes down to workshops (normal not magic), the more you have the more orcs you need. Its simple maths just times your workshops by so many thousand orcs (wiki helps). From memory 2,000 ish per upgrade so 4,000-5,000 per chapter per workshop. If you naturally allow for a buffer you will be fine.
As to orc nests don't they only start producing orcs from chapter 8? I thought prior to 8 they are supplies or goods. So you would need to get them in chapter 8 or have restoration spells for them. They can be crafted in MA I believe, but anything decent is hard to come by there.