Is there a particular number of expected sales that an agent would have in mind before agenting a non-fiction writer?
Non-fiction writers don't need any sales at all to get an agent - in fact, they don't even need a manuscript. The hunger for good non-fiction is so great amongst publishers and, thus, agents that all you need is a good idea, a good proposal and some proof that you can write. It helps if you're a journalist - because you can point to a body of work, an ability to meet deadlines and an acceptance of the editing process - but it's not essential. It's also helpful if you're an expert in something if you're writing about health or science or gardening. If you're a previously published writer, that's nice too - but none of it guarantees you'll be considered by an agent or guarantees that you won't. It all comes down to what you're writing about and how you write it.