Start off with the 12 or so as others have mentioned, but later, add another 10 to 12 and keep track of each link site on things like:
1. Do they approve your sites?
2. Do they inform you of anything that needs attention?
3. Are they quick with their reviews and approvals?
There's more to the list but generally you want to create a good relationship with link sites, but also want to know your sites will be reviewed and approved in quick time. Think of it as customer service...for some its just a hobby, but generally its a business
