How many credits is enough to query a partial?

You never need to write the whole book in advance for nonfiction. You write a book proposal and three sample chapters. Search around some of the threads in this forum and you should find a long discussion of proposals and how to query nf.

Credits

There is no magic number of credits. Big, national credits can make an editor pay closer attention, but any credits at all puts you ahead of many writers. The main thing is to have a good idea for teh book, and to write a proposal and sample chapters that are good.